Is he powerful in MGS4? I know he controlled the B&B's, but that's similar to him controlling Meryl. I don't think he doesn't anything on par with creating a gigantic fire whale in MGS4.
Well he managed to control an army from beyond the grave. It was a big step up from MGS1. It took The Sorrow to intervene which is a whole different can of worms.
As I said before I just see it as Mantis in TPP being at his prime level of power. This is the Mantis who has been experimented on and is having his powers trained by a Russian organisation, not the far more laid back adult Mantis who chooses to manipulate people rather than destroy them.
Except there's no explanation as to how his prime is apparently as a child and dead but not as an adult.