I finally got around to trying an Nikon F4 VF focus-screen in an F3.
I had a "red-dot" F3 "E" screen in one F3 HP body, and an F4 "B"
screen in another F3 HP body (the screens are similar-type, except
for the "E" having a grid, and the F4 "B" having AF markers), with
50mm f1.8 Nikkors on both bodies (I switched lenses, to eliminate
the unlikely influence of a particular lens on the observations).
Though the screens appear slightly different in hand, when installed
in the F3 bodies and compared side-by-side, I could detect little
difference in either brightness or sharpness with the two screens.
When I tried the DA-2 action finder, though, I found the F4 screen
to be noticeably more difficult to focus with than the F3 screen.
BTW, screens seem to be optimised for lenses of about f2-f4, being
a bit harder to focus with (manually, using the "ground-glass"
surface) if the lens is either faster or slower. And a brighter
view is not necessarily a better view for focusing.
Also, BTW, I find the F3 finder preferable to the F4's - the F3
finder is sharp all-over, while the F4's VF is not as sharp on the
edges, and the unsharpness is not equal on both sides of the VF
screen (maybe due to the variable-focus eyepiece, and decentering
of its optics due to a problem with the eyepiece focus mechanism).
Hope This Helps