Whist you can 'choose' your exit date through PVR, the 'system' does NOT have to accept it and can, as was my case, choose their own PVR date! If they elect to let you go ealrlier than your request, and 'their' date falls within your 4 months window, then you are stuffed, big time!! So do be careful.
That was the danger that stood out to me. I can't see anyone doing it maliciously to a serviceman to stuff them over pensions provision, but for many reasons, the potential is there for it to happen.