SS
Was in a similar situation myself when I called it a day.
In my case, it wasn't so important re pension etc. but the pricipal is the same. The important date is (or was in my day) your LAST day of service but..................................... and its a big but!
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.
I think I would do the 4 months, safe in the knowledge that you will be finacially screwed.
Winco