The system requirements of the engine are explained in the FAQs, but the requirements of the games are not specified.
How can I specify them? Do particles, shading, physics or other things influence requisites?
For example, Goat Simulator (UE game) works on AMD radeon 1220 (UE requires 8xxx).
It depends on the game. UE4 is designed to eventually work with every platform. There’s even an experimental HTML setting in your preferences. You need to look at the MAXIMUM specs for your given device if your worried about playability.
You can controle how good it will run on a PC with the quality settings. 
Also take a look at this thread: https://answers.unrealengine/questions/36895/what-are-the-system-requirements-for-games-that-we.html