The difference from Unreal 3.9 (shown in Samaritan tech demo
) to Unreal 4 isn't really a huge leap as people might expect. I mean sure Unreal 4 was written from scratch to give a better performance on future hardware/future APIs (eg. DX12) and using all the visual techniques that are present from unreal 3.9. DX11 MSAA Hair, DX11 Subsurface Scattering, DX11 Enhanced Bloom, Apex Clothing (physics), HD shadows, Bokeh depth of field, Shadowed point light reflections, DX11 Tessellation and displacement, DX11 Deferred rendering with MSAA support, DX11 Image Based reflections, real-time colour grading, Cascaded shadow mapping and much much more. There's also the new tools for developers to make it more efficient then before.
“Reality is that which, when you stop believing in it, doesn't go away.” ― Philip K. Dick