There is no separate "version." It's all just one single engine.
The engine just has lots of different settings and features that can be turned on or off at will. When you create a project the editor asks you if you wish to target mobile or desktop, and choosing mobile will make the engine automatically turn off/on certain settings for you to maximize performance (but you can actually change those settings yourself manually later). You could tell it you're targeting desktop, yet then go and change settings yourself to mobile-friendly ones if you want your game to run on super low-end PCs.
Many mobile games, for example, completely turn off all lighting features of the engine and use only unlit materials (as in, all lighting/shadowing, if any at all, is permanently etched into the model's texture, not calculated in real time).
Since the difference between UE4 mobile vs desktop is just changing certain settings, the engine itself is not the biggest obstacle for porting. It's the assets and how they're created and optimized that matters most (you wouldn't want to use super hi-poly models in a mobile game) and that's something performed in Maya, 3DS Max, Blender, or whatever 3D modeling program developers happen to use. The other obstacle is control input methods. Porting a touch screen game to a console with only a traditional controller would require rewriting a lot of code.
