Morris definitely seems to have filled that last 'big' role, but I suppose Monroe could be a better option than Theis. The thing about Theis is even though he isn't a particularly great playmaker and I don't love his post game, he does exactly what you want your 4th big to be able to do: bring energy, rebound, block, even knock down an outside shot.
I'm afraid Monroe is just going to be a worse version of Horford/Morris/Baynes - although I suppose he has the potential to be very good, as he once was. Theis brings something totally different.