Free to use unless you sell, with any games made using UE4 having a 5% levy on any sales.
https://www.unrealengine.com/what-is-unreal-engine-4
Not sure what this means for FOSS. It's probably a bad thing, if anything, for the FOSS engines such as OGRE but on the flip side it probably opens up the way for FOSS games based on UE4? Although if your FOSS game uses a proprietary engine, is it truly FOSS? (Well, no its not, but how many aspiring game devs will care about that?)