Thanks for replying. I checked the S and P modes when pointing the lens to a bright lamp checking this way if the lever is stopping down and it's all working as it should: bright light in S mode yields in a small aperture at 1/8sec, aperture is almost max when pointed to a dark area.
So I still believe after experiencing the big differences between F100 & F4 at long shutter times of 1,2 and 4 secs-up to 50% less for the F4s on 2secs - that my shutter unit isn't working properly in P and S modes. Why this happens is the question now.
Strangely the A and M modes are firing away just perfect.
I've got another F4s body for 90$(without prism) or 250$ for a complete unit. So maybe I have to head towards one of these. I just love the grip and the weight of this camera too much. Old School "VR", 1,5kg lens and body in your hand.