The short answer is: GT4 is not a true to life simulator.
The explanation is:
A factory car dropping boost pressure only happens in cars with turbochargers that are too small. The boost pressure starts to drop when a turbo simply cannot flow any more air, and the pressure starts to drop as engine RPM's increase. This happened to my Nissan 300zx Z31 with a boost controller, but it does not happen to the factory car. Stock boost pressure is 6.8psi. With a boost controller installed on the factory turbocharger, the turbo hits 14psi at 3200rpm and drops off to 10psi by 6000rpm. Removing the exhaust completely gave me 12psi at 6000rpm, and fitting a bigger turbocharger gave me a constant 14psi from 4000rpm till rev limiter. (at a guess, the turbo I fitted would be roughly equivalent to stage 2 in GT4, but real turbos don't have "stages")
Fitting a blow off valve, diverter valve or pop-off valve (all are totally different components) will not make boost drop off at a certain rpm, but they can limit the boost to a certain PSI. My 300zx had a pop-off valve that limits boost pressure to 8psi to avoid engine damage. I had to remove this when installing the boost controller.
I have noticed the boost drop-off in GT4 happens in the Golf V GTi and the Australian Ford Falcon Typhoon 4L i6 Turbo, and I can definitely say boost pressure does not drop off in a real Falcon turbo.
The boost pressure dropping off on standard cars in GT4 is somewhat realistic, but it should not drop to 0, and it should not drop at all on any car fitted with an upgraded turbocharger.
No, I'm not a mechanic. A mechanic wouldn't know most of this stuff anyway.
I used to work in a turbocharger shop.