I don’t think any of the companions acted
that out of character in Four to Doomsday; Adric was always ready to side with the bad guy (see State of Decay, Snakedance…hell I’m surprised in Earthshock he didn’t say “Hmm, you know I think you Cybermen might be on to something…”) as for Tegan, this is still a very early adventure for her so the character isn’t fully formed, but hell in her place I might have made a run for the Tardis as well…and I could be mistaken (though it’s not that long since I saw it) but wasn’t part of her reasoning that she had to warn Earth?
Nyssa not having much to do…see most Fifth Doctor stories