As if a two-in-one wasn't enough, Asus has gone down the road of "four-in-one" with the Transformer Book Duet TD300. Part laptop, part tablet, all Windows 8.1 and all Android - is it the ultimate device without compromise or a confused take on the multiples concept?The TD300's key sell is the Dual-OS concept. It's not Android built on top of Windows or vice versa - the press of an icon in either of the operating systems will switch to the other system in just four seconds. If you want to use Windows for work and Android for play, it's up to you. The icon that activates the switching is easily visible as a Windows tile within the base of the desktop, and as an Android icon in the main tray for quick access. It really is as simple as one press.\u00a0As a laptop, with Windows 8 up and running, the device feels solid. The keyboard dock is finished well, comfortable to type on, and the trackpad smooth to the touch and responsive. The screen resolution comes in two options and you will want to opt for the 1920 x 1080 IPS panel as it looks sharp and has decent viewing angles. By comparison the non-IPS lower resolution panel is poor for the price, as the viewing angles are not good. It's HD all the way or no go, but the better panel will cost more.It's set up as a tablet by releasing it from the keyboard dock with the press of a button - although it's a fair fiddle to extract the screen - and then\u00a0flipped into Android or Windows, as you choose. Our issue here\u00a0is twofold: there's the sheer size of the product - at 13.3-inches that screen is large and at almost 13mm it's bulky too - so we just wouldn't see ourselves using it as a tablet device; and it's Android 4.1, which is well behind the current standard.It's size and weight that will split the crowd on this one. And that's an important factor to consider because the\u00a0combined weight of\u00a01.9kg isn't small fry. As a laptop replacement it's a reasonable offering, but as a tablet it doesn't tick the boxes for us at that scale.\u00a0Under the hood, though, there's a lot of power. A fourth-generation Intel Core CPU - which one depends on how much cash you're willing to fork out, as an i7 won't come cheap - works alongside 4GB or DD3L 1600 RAM. In our short time with the product we weren't able to delve deep to see how well heavyweight programs pushed the power available, but the running of each OS was smooth from what we saw.Potentially powerful and an interesting concept - but we'd like to see a trim 11.6-inch model before we part with our cash. Current start price is $599 (\u00a3365), but add in the HD IPS panel and better processor and we suspect that will soar after some spec personalisation.