I am having a hard time to decide how to do this. Should I make a terrain and paint the field on in UE4, or should I use a 3D asset software to make the field and stadium?
I think this depends on how much detail you need. Are you going to be able to walk on it and interact with it?
If it’s just for something like an aerial view or something, I would just create a 3d plane and texture it. If you are going to be able to walk on it, I would create some terrain and use terrain tools create it and place foliage (the turf sticking up) on it.