I don't think the turnover of characters helped, because a lot of the time in series 3 you ended up with characters being shoved into roles that had previously been occupied by characters who got killed off or left, and there was a bit too much shuffling around of roles.

Abby in particular got messed around, because she ended up spending a lot of time in series 3 filling the roles that Stephen and/or Cutter used to fill, while being simultaneously pushed out of the action side of things by the arrival of Danny.

I'd go so far as to say that the whole team dynamics got cocked up in series 3, and I think the main reason behind that was the fact that they replaced a scientist team leader with an action man team leader. That substitution caused a lot of cocking about with the team roles of previously established characters like Connor and Abby. With Matt as leader they sort of went for the best of both worlds - a cerebral action man - and that put a bit of science/action balance back into the team.

That said, I do agree there was some general all-purpose silliness and out of character behaviour in 3 and 4 that made no sense at all, even give the need for a bit of role reshuffling.