UE4 uses an entirely new method of rendering materials (we now use the full material node graph that you see in the Material Editor). In UE3, we had hand written shaders that were rather limited (we managed to squeak out some pretty decent visuals, but it wasn’t easy, and it was counter-intuitive for most artists/users).
Epic Citadel was doing a lot less on the GPU, for better or worse. That said, things like Tappy Chicken are not doing much either, even in UE4, so have the ability to run on lower devices.
We are going to be focusing more on the lower end devices going forward. However, we are limited by the state of the drivers on the devices, which we are discovering are not all well optimized for this level of content quality. We are and will be working with GPU manufacturers to find ways to maximize performance, hopefully without reducing the quality very much.
I hope this helps explain the differences,
Josh