Which fast (MF) Nikon tele lens?

Hi folks,

now that I more often do have the problem that my Nikkor 400/5.6 isn't
fast enough, especially when being combined with a teleconverter, I
want to switch a faster lens of this kind. Only problem I haven't
solved yet: Better buying the 400/3.5 of the 500/4.0.

(Since I am only using F4s bodies, missing P-chip is no problem)

Any advice on both lenses would be very helpfull.

Greets form Germany

Thomas