Sony has quietly announced its next flagship, the Xperia Z4 smartphone in Japan. The device has been rumoured for several months and after it didn't appear at Mobile World Congress like some expected, many were speculating a release later this year.

Although Sony has yet to detail when and which other countries the Xperia Z4 will come to, it has said it will be heading for Japan in the summer. We have crunched the specs and put it up against the current Xperia Z3 flagship to see how they differ and what you can expect to change with the Xperia Z4.

The Sony Xperia Z4 brings with it the same OmniBalance design we have come to expect from the Xperia Z range, but Sony has refined it yet again with the new handset.

You'll still find the side power button, aluminium frame, tempered glass rear and rounded corners, but Sony has opened up the Micro USB port and removed the cover. This is the same as what it did for the Xperia M4 mid-range device that it launched at MWC back in early March. The Xperia Z4 will still be waterproof like the Xperia Z3 but it will be more convenient to charge.

In terms of footprint, the Xperia Z4 measures 146 x 72 x 6.9mm, while the Xperia Z3 measures 146 x 72 x 7.3mm so the new device sheds a few pounds off its waistline and it also drops in weight from 152g to 144g.

The Xperia Z4 has a 5.2-inch display that sports a resolution of 1920 x 1080 to offer a pixel density of 424ppi. This is the same as the Xperia Z3 so you shouldn't notice any difference in the two devices when it comes to the screen.

Despite many rumours claiming the Xperia Z4 would step up to a Quad HD resolution like many of its competitors, this hasn't turned out to be the case. Sony has stuck to its Full HD resolution guns for the last couple of Xperia Z smartphones now, claiming battery life is more important, but some may be disappointed to hear the Xperia Z4 doesn't offer a jump up to match the likes of the Galaxy S6.

Sony's Triluminos, X-Reality and Live Colour LED techologies were all on board the Xperia Z3 so we would expect these to also be present in the Xperia Z4.

Sony has consistently marketed its Xperia Z range with the camera at the forefront and so it will come as no surprise that the Xperia Z4 offers the 20.7-megaixel rear snapper of the Xperia Z3.

It's not specified in the specs released by Japan, but we would expect the same maximum ISO 12,800 sensitivity on the Xperia Z4 that was found with the Xperia Z3, as well as auto focus functionality.

The front-facing camera of the Xperia Z4 does take it up a notch from the Xperia Z3 however, coming in at 5-megapixels compared to 2.2-megapixels. Not much more is given away but we would expect the same software features as the Xperia Z3.

Under the hood, the Xperia Z4 features a 64-bit octa-core Qualcomm Snapdragon 810 processor, which is a step up from the Qualcomm 801 processor found in the Xperia Z3.

Both offer 3GB of RAM but the Xperia Z4 doubles the initial internal storage compared to the Xperia Z3. The Xperia Z3 comes with 16GB flash memory, while the Xperia Z4 is listed as 32GB. Both are expandable via microSD up to 128GB.

So far, it doesn't seem like much has changed with the new handset, but interestingly, the Xperia Z4 offers a smaller battery capacity than the Xperia Z3. The Xperia Z4 will come with a 2930mAh battery compared to the 3100mAh capacity found in the Xperia Z3.

As you would expect, the Xperia Z4 will come with Android 5.0 Lollipop straight from the box and as the Xperia Z3 has now been updated to the latest software, you shouldn't see any difference in terms of user experience here.

Sony's skin will still be over the top, offering easy access to the Walkman, Entertainment, PS4 Remote Play and Xperia Lounge.

The Xperia Z4 makes small improvements in numerous places over its predecessor, but it doesn't do anything drastic like Samsung did with the S6.

You get the same design but a little lighter and slimmer, and you get the same 5.2-inch Full HD display so you won't notice a great deal of difference to begin with, except for the open Micro USB port.

Go a little deeper and under the bonnet however, and the Xperia Z4 increases its power and internal storage, as well as improving its front-facing snapper. It does drop the battery capacity down however, and there will be no variation on software between the two devices.

In a nutshell, the Xperia Z4 makes improvements over the Xperia Z3 as you would expect from a succeeding device, but the difference between the two is pretty minor. The Xperia Z4 would win the numbers game but Sony has refined its already decent flagship, rather than given it an overhaul. Think of it a little like what Apple does every other year when it adds an "S" rather than give its new smartphone a new number.