inside the turbo the air mass will be greater in a T-4 than a T-3 at the same air pressure.
What I'm trying to tell you here is that inside the plenum chamber the air mass/volum will be the same nomatter what pushes the air in at the same pressure.
the volum in the plenum won't be bigger because you change turbo.

a T-4 on 1 bar boost gives a lot more air than a T-3 on 1 bar, but when you pass this air into a intercooler and throttlebody with smaller inside diameter two things can happen. if the head flow capacity is good enough - the air velocity will increase.
if the flow capacity is already on the limit - boost will rise.
simple as that.
this is the reason why boost drops on high rpm with a full stage 3 setup and a T-3. the turbo can't deliver enough air to maintain boost pressure.