The Android devices tested at Epic currently:
https://docs.unrealengine.com/latest/INT/Platforms/Android/DeviceCompatibility/index.html
Larger list from the community:
UE4 mobile needs more advances. I hope it becomes versatile enough on the low end. Ideally, it would run lightweight games (e.g. chess, puzzlers, Bejeweled style, etc.) on lesser devices – requiring only what’s expected for a less demanding game – so I needn’t resort to using an additional engine (Cocos2d, etc.) to support the average touchscreen user worldwide. I’m hopeful. We’ll see how the lists improve.