Is it possible to mod games using UE4?

Hello! I have one quick question, is it possible to mod games within Unreal Engine 4? Or am I limited to using just the Unreal Development Kit(UDK) and/or Unreal Tournament(UT)?

In further detail, I would like to mod a game called Goat Simulator on the PC, but it was built off of UDK 3. By default, the actual game comes with a program called Goat Simulator Development Kit(Which is also run off of UDK 3). I do not wan’t to be limited to UDK and, if possible, would like to use Unreal Engine 4 to mod the game. Is this possible?

No, UE4 is not compatible with UE3.

Thank you for clearing things up. I was very curious on this subject.