>But is the AF F4 screen useable for MF focussing (most of them are
>very clear, but are difficult to focus manually)

In my brief experience with it, it did seem good for MF.

>And I wondered if the different viewfinders of the different bodies
>can also be exchanged ??

The F/F2 prisms and WL finders can be exchanged if you are willing
to remove nameplates, but the Photomic heads will not exchange, and
I think there is no head compatibility between F3/F4.
David