Is there a specific reason you need UE4 to run on an ultrabook? Especially in that price range you’ll be lucky to end up with something that might just be able to start the editor, but do not expect anything to work fast unless you limit yourself to tappy chicken style games.
A slightly thicker gaming oriented laptop should provide a much better experience, but if budget is your main concern you can probably buy a desktop for that money that will be faster than any $ 2000+ laptop.
Anyway, if there is a specific reason you need the portability your main concern will indeed be the GPU. A good place to compare them is notebookcheck.net. Here’s their main comparison list of mobile GPUs.