Unreal Engine 1 was out in 1998
Unreal Engine 2 was out in 2001, with the code core and rendering engine completely rewritten
Unreal Engine 3 was out in 2006
Unreal Engine 4 was out in 2014
From UE3 release to UE4, it took about 8 years. Unlike other engines, that in their next versions they keep their base features, each version of Unreal Engine is a completely a new engine.
Now, as you could see, Unreal Engine 4 gets updates, named 4.1, 4.2, 4.3 . Each one is released after a few months. In a few years, UE4 will reach the version 4.9 . After that, the engine will be named UE5, like in the case of Unity, or Unreal Engine 4.9764 , 4.9765… . Will Unreal Engine 5 be different than UE4, or it will remain mainly the same, with updated features ?