Believe it or not i found a ONE track mixer to be the best for my needs.
i used a frontier Alpha track.
mainly i used it for riding music or atmos tracks under dialogue.
it has a dial for selecting which tack you are controlling, so very fast & easy to use.
i dont think they make it anymore 🙁
the other mixer for FCP i had was an 8 track behringer.
it was too big and cumbersome for me,
and the plastic faders made an awful noise clacking up and down in automation mode
im not aware of a four track option.
nick