Right now UE4 is using a cluster f*** of different techniques made by different arcane artists for different purposes.
Static lighting is handled by lightmass, Epic’s own solution
Realtime lighting is simply direct lights and cascaded shadow maps
Realtime distance shadowing and sometimes interior shadowing is handled by Distance Field Soft Shadows
Realtime skylights are shadowed by distance fields as well
Distance shadowing for landscapes uses something else entirely
Realtime GI is in a beta state, with 2 native techniques:
Light Propagation Volumes, a quick and dirty yet somewhat effective technique made by lionhead
Distance Field GI and Height Field GI are available as a WIP in 4.8. DFGI is for static meshes, HFGI is for terrain
UE4 is simply the most well organized mess of pentagrams written in C++ you’ll ever see.