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
I didn't mean that any of them were acting out of character, I haven't seen enough of Tegan or Nyssa to be able to judge that, just that they were being poorly utilized. At this point Tegan and Adric are annoying me and I'm just nitpicking.