Anti-aliasing in Unreal Engine 4 is performed in post processing using FXAA, which is an efficient GPU MLAA implementation.
This method solves most causes of aliasing artifacts, but cannot fully prevent temporal aliasing.
Other techniques: UE4 currently supports MSAA only for a few editor 3d primitives (e.g. 3D Gizmo) and Temporal AA.
The feature can be disabled by the show flag Post-process Anti-aliasing.
For more information, see: FXAA Whitepaper (external link)