Umberto, I think you are under the impression that FSX and P3Dv4 are both on the same computer. They are not; FSX and P3Dv4 are on two separate and different computers.
It doesn't make any difference. They would still install the same set of files (except *just* bglmanx64.dll which is not used by FSX, and this has no relationship with jetways) and, nothing in the different system could cause jetways not working AT KLAS ONLY, which is what you are reporting.
Unless, of course, you haven't:
- installed the same KLAS on the two systems
- installed the same GSX on the two systems
- tested with the same airplane, in the same position, at the same gate, on the two systems.
One of these MUST be different, because it's the only possible explanation of why jetways are not working at KLAS, on one system only.