Havok Physics it’s not free, isn’t an option to integrate with UE4. (But can be added like Speedtree or other solutions)
You have examples of different physics engines in different well know games:
-
Bullet Physics:
Grand Theft Auto V - IV -
Havok Physics:
Orcs Must Die 1 - 2
Far Cry 3 - 4 -
PhysX
Borderlands series
Metro
The 90% of Unreal Engine based games
And others with custom ones:
BeamNG
Crysis
Each physics engine have different pros & cons.