Reddit mentions of aLLreLi 4K UHD 4 Port (1x4) HDMI Splitter w/ PIP (Picture in picture) - 4 Hd Input 1 Output Full Ultra HD [V1.4 Certified] 1080p & 3D Resolutions Switch for PS3 / Xbox 360 (slim) / Sky HD / Freesat HD / Virgin + / Bluray Player / DVD / HD Camcorder / HTPC / Laptop

You're looking for Picture in Picture (PIP)? You can buy a device for under $50 for this, however the major drawback is that the screen won't be evenly split between the inputs. Like traditional PIP, there is one main program that encompasses the entire screen and the sub-programs are overlaid on top at a fraction of the size. You can see this in the sample photos in the links below.

http://www.amazon.com/aLLreli-Premium-Certified-Splitter-Picture/dp/B00M0HF2TU

http://www.amazon.com/Etekcity%C2%AE-Switch-wireless-adapter-Supports/dp/B00H9642JS

I think you should forget the whole PIP/split screen thing and just get 2 displays. You'll have way more control over your inputs.

Boom, less than $20, just need any old 4K screen. There are plenty of little boxes like this that do just what you want, and no need to look for a screen with the functionality at all.