We have 100% access to Unreal Engine 4 Code, but it is not Open Source. so is it legal if we fork it, heavily modify it and re-brand it ?

I suggest read your license in that case. I believe that this is not allowed at all.

Probably not. Unless by heavily modify, you mean get ideas from their code and build your own from the ground up. There have been court cases won by companies claiming the accused is infringing on copywriter even over single lines of code. (This example is obviously the extreme) IMO talk to a lawyer before you sink to much time into it, if you are going to go the heavily modify approach.

@hexc they also lost some of I'm right

If the license you got says you’re allowed to, then by all means do (if needed)!

I'd just save yourself the ball ache, use godot and add what you need

@dufferz just asking if it's possible. im no expert in c++ either.