HTC Sense 4.0 is the latest version of HTC's user interface, layered over the top of Android 4. It lands on the HTC One series, as seen the HTC One X reviewed here.Sense 3.6, on the other hand, is something of a halfway house between "old Sense" and the newer cleaner Sense. If you're interested in HTC Sense 3.6, we gave it a thorough reviewing here when it landed on the HTC Sensation XE.\u00a0HTC Sense 4.0 will be present on devices moving forward (until HTC moves on again) with Sense 3.6 landing on devices as they are updated to Android 4.0, Ice Cream Sandwich. We've asked HTC whether it will be bringing Sense 4.0 to older devices and that doesn't seem to have been determined yet; we suspect not.So, if you have an existing HTC Android smartphone due for upgrade to Android 4.0, it's almost certain you'll receive Sense 3.6. Here we'll walk you through the main differences between the two versions, so you can see what you're missing out on, or not.Sense does differ from device to device: here we are reviewing Sense 4.0 on the HTC One X and Sense 3.6 on the Sensation XE. In the sections below when we're comparing images side by side the Sense 4.0 version is on the left.Home screenMuch of the upfront identity of HTC Sense comes from the home screen. That iconic weather clock has been in place since Sense launched in 2009\u00a0on the HTC Hero. While the widgets have changed slightly with each iteration of Sense, the effect is very similar, but so much more refined now.\u00a0With the old launch bar gone, both Sense 3.6 and Sense 4.0 offer a customisable launcher at the bottom of the home screens. They both work in the same way, letting you add shortcuts as you fancy, and are very close to\u00a0the\u00a0native Android 4.0 arrangement. The design is slightly different, but it is essentially the same.With both versions of Sense you can have up to seven home screens to customise. One minor difference is that Sense 3.6 will rotate on a carousel, ie, you get to the far left and it rolls back round to the far right. Sense 4.0 only travels from one end to the other, so you'll have to swipe back from one end to the other.WidgetsPress and hold the wallpaper and you'll be taken through to the "personalise" screen on both UIs. On Sense 3.6 this is the old menu, offering access to widgets and so on and the option to tab over to display changes (wallpaper, skin, lock screen) as well as your sound setup.\u00a0\u00a0In Sense 4.0 this menu offers only widgets and shortcuts, but it's much more cleanly done. The new version gives you a better interface, letting you drag widgets and shortcuts on to the relevant pages. We much prefer this clean and visual approach, but by contrast, if you want to tinker with other elements, you have to head into the settings menu. That said, just how often do you customise your phone?\u00a0The other thing to note about widgets, and in many case the menus, is that they are now lighter, with HTC losing the dark backgrounds in many cases. Obviously, this may differ with devices and themes, but we prefer a white background, with black text.The signature HTC weather clock widget is lighter too, losing the background and refining the detail. We've also noticed that it's much smoother when moving from screen to screen. In Sense 3.6 the weather clock doesn't rotate smoothly over the screen, it sort of starts rotating, and then slides off at a different angle. Also, the Sense 3.6 version has unnecessary depth: the side showed details that have been ditched in Sense 4.0, thankfully.HTC Sense 4.0 widgets can also be resized now, something that wasn't possible in Sense 3.6. As\u00a0Sense\u00a03.6 landed on Ice Cream Sandwich, some widgets can be resized, but in Sense 4.0 this extends to some of the common HTC widgets, like People or Friend Stream.Lock screenBoth Sense 3.6 and Sense 4.0 offer lots of lock screen functions and they both work in the same way. First up, you get the unlock options which will pick up whatever you have on the launcher bar and put these on the lock screen.\u00a0You can't choose the apps you'd like to have as unlock options, as you could in the previous version of Sense, but it's likely you'll have the apps you use most often here anyway. If you have security enabled - pattern, PIN or face unlock - you'll still have to go through that process before you land on what you wanted.\u00a0In both you get to choose the background, so you can have photos, stocks or weather, although Sense 4.0 now gives you eight options, with the addition of productivity and people into the mix for more lock screen information.As this is Android 4.0, you'll also be able to access the notifications bar in both versions, so you can instantly get to alerts and take action, which we'll discuss in a second.\u00a0You can also change the ringer volume using the rocker from the lock screen in Sense 4.0, which you can't in Sense 3.6. As previously, once the volume slider comes up, you can tap the icon on the screen to switch the phone to silent.NotificationsThe notifications bar has seen a few revisions in Sense 4.0 from Sense 3.6. The biggest difference is that the dual-tabbed layout in Sense 3.6 had been removed and the recent apps are now gone. We've noticed that the notifications bar is much quicker to open in Sense 4.0, although this may purely be down to the power of the HTC One X.\u00a0One common feature is that both support the swiping to clear notifications, so if you don't want to read your emails, you can just swipe away the alert.\u00a0Both give you music controls, as well as the route to access Beats Audio, if you have a Beats device. The new One series are all Beats phones, but only the Sensation XE and Sensation XL were previously.The other slight change that has come about by removing the two tabs in Sense 4.0 is the addition of a Settings button in the notifications area. This is the fastest way to access settings (unless you drop a shortcut on the home page), and beats the Sense 3.6 version which wants you to switch to the quick settings tab, then press the additional settings icon to land in the same main settings menu.Naturally, as HTC devices upgrading to Android 4.0 have a menu button, this will give you additional options that the One series Android devices lack, as they have the standard three Android 4.0 controls. As such, you can access the settings menu in Sense 3.6 from the home screen via the menu button.Apps trayPressing the central apps icon on the launcher opens the apps tray. The biggest change here is that from vertical scrolling to horizontal. In both cases you still have three tabs at the bottom offering all, frequent and downloads.\u00a0In Sense 4.0 you can reorder these, so if you want to open on frequent rather than all you can make that change. The other thing that Sense 4.0 adds is a selection of shortcut icons across the top of the apps tray.These offer you direct links to Google Play and searching, so you can quickly search for apps within the tray. You can search from the search button in old devices, but in the move away from physical controls to on-screen controls, we prefer the newer approach.\u00a0Recent appsOne of the highlight features of Ice Cream Sandwich is the addition of the Honeycomb "recent apps" feature, which makes multitasking (or task switching) much simpler and cleaner than the old long-press-on-home option.\u00a0Both Sense 3.6 and Sense 4.0 integrate this feature but it's visually very different. In Sense 3.6 you access the feature via a long press on home (as before) because there is no direct "recent apps" button. Sense 3.6 looks the same as native Android, with thumbnails you can scroll vertically, swiping away apps you've finished with.On Sense 4.0, the list scrolls horizontally and the grabs are much larger. It will also rotate (after a fashion) so you can view in both landscape and portrait and again, you can swipe things off the top or get rid of them. Sense 4.0 is definitely more glossy, but with Sense 3.6 you get more on the screen, so might be faster to navigate.KeyboardThe Keyboard has seen a change visually between Sense 3.6 and Sense 4.0. The old grey on grey of Sense 3.6 and previously has been replaced with an altogether darker arrangement. The key characters are now white, which perhaps makes them easier to see.\u00a0The keys also fill slightly more of the space available, with less background on show, but otherwise offer the same predictive arrangement. You get access to HTC's Swype-alike trace entry too, as well as the integration of voice.\u00a0However, the Sense 4.0 One X keyboard includes cursor keys across the bottom, which we've found superfluous, as we've never needed to use them and they take up space on the display that could be better used. And you wouldn't want to miss that horse sale now would you?BrowserThe Sense 4.0 browser has been tweaked slightly from Sense 3.6, adding a new "read" option to the top and a run of action buttons across the bottom.\u00a0The odd thing about Sense 4.0's read option is that it doesn't always appear. We haven't quite figured out exactly what triggers it, but when it does appear, it will let you view a page in simple text, without the images and adverts, very much like the reader view in Apple's Safari browser.The other option that Sense 4.0 brings is a reading list, giving you the chance to save and read later. This can be accessed via the menu or through the "add to" action button at the bottom of the page.\u00a0The other action buttons - which hide during normal browsing and appear when you swipe down when at the top of a page - offer access to bookmarks, saved pages and tabs.\u00a0Tabs in Sense 3.6 are called windows, but work in very much the same way, with both versions offering up to six open tabs at any one time. As Sense 3.6 lacks the bottom access buttons, everything is accessed via the menu button at the bottom of the phone, which opens a menu very similar to Sense 4.0's in-app menu button.\u00a0Apart from that, the only other significant difference appears to be the option to enable or disable Flash in the browser in Sense 4.0. Otherwise, both offer Incognito tabs and a desktop viewing option.\u00a0Music and mediaMusic in Sense 4.0 is a hub rather than a standalone app. It means that all your music apps and services can be accessed through one place, although in reality, you're just adding shortcuts into one area for convenience.\u00a0In the actual music player things are slightly different visually, but the functionality is largely the same with the ability to make playlists and share details and so on. Both music players give you access to network music too, so you can retrieve from a sharing PC or media server.\u00a0Elsewhere you'll find the Gallery app is basically the same on both Sense 3.6 and 4.0, although the new version also offers Dropbox and SkyDrive integration. Both versions of Sense offer sharing with external devices: Sense 3.6 has a button for this in the viewing window and Sense 4.0 would rather you trigger this using a three-finger swipe up the display.In reality the system is very similar, as you'll end up at the same screen to select the external player you'd like, if not already paired. The same applies to the Gallery video player, although Sense 4.0 adds the option to take an instant screenshot from any video you're watching.\u00a0One of the things that Sense 4.0 does away with is the Connected Media app. This app in Sense 3.6 (and previously) was really just a collection of links through to the sharing options of the Music and Gallery apps. It's one less thing to clutter up your apps tray and we're glad it has been removed.\u00a0Camera interfaceThe camera interface has undergone a substantial overhaul in Sense 4.0, perhaps one of the areas that has seen the biggest update. What it really changes is the need to access little pop-up menus to switch main functions, presenting both still and video capture buttons on the screen at all times.The range of effects is still accessed via an on-screen button, but other controls have switched sides, so in Sense 4.0 you'll get the settings, flash and shooting mode options on the left, rather than piled in together with the shutter button on Sense 3.6.It opens things up a lot and makes the app feel much nicer to use. It also clears the options away from the most important buttons, which we like.\u00a0However, many of the options are very much the same, with plenty of things to tweak. We're not so keen on the lack of a focusing reticule on Sense 4.0, as you never really know what it has focused on.Focusing and capture is much faster in Sense 4.0 although this may differ from device to device and you now get a burst shooting mode. To trigger this, all you have to do is leave your finger on the photo button and it will rattle off shots continuously. You'll then be asked to select the best, which you'll keep and the rest are dumped, great for catching fleeting moments of action.Other apps and services, conclusionsHTC Sense incorporates a lot of services and features and we can't pick them all out here, but there are some new additions.There is now a new Movie Editor app, which takes things further than the previous trim options of Sense 3.6. This will let you pile together video and photos into different themes to create a movie on the fly. It's nicely done, giving you the option to add your own music and save at different resolutions.You also get a car dock mode, which gives you all the pertinent apps for driving, with nice big icons. This can be set to launch automatically\u00a0when connected to a car dock, something we've not yet tested. It will also let you decide on your default navigation app, which is a win for those who want to escape from HTC's Locations.\u00a0Overall, Sense 4.0 is a more complete and more authentic Android 4.0 experience. Sense 3.6 makes some big changes from previous version of Sense, but Sense 4.0 takes things a step further, removing clutter, adding convenience and, in some cases, stepping closer to native Android.As always, there will be lots of little tweaks and differences throughout the Sense experience of both versions but here we've outlined the main areas and the biggest differences.\u00a0If you're in any doubt, we feel that Sense 4.0 is the better. Sense 3.6, by comparison, does feel a little like a stopgap. However, Sense 3.6 is a great step forward from older versions of Sense, making it slightly less oppressive and slightly more flexible.To find out when to expect an upgrade, why not read our guide to Ice Cream Sandwich updates. If you're sold on Sense 4.0, then check out our full HTC One X review!Any questions? Let us know in comments below.