I was looking at the formations Brentford used over the last season and they have two clear formations - 433 when they think they can beat a team, and a more defensive 532 when they're playing one of the big boys. I also remember Man City changing their formation to 3 at the back when they played us in our first season back in the Premier League, so I don't think it's out of the question that a team shouldn't have a variety of formations to suit a particular situation. I also think 433 or 4231 is the 'modern' way of playing if you want a chance of beating the teams you 'should' be beating.
Unfortunately for us, everything from training to recruitment is geared to one way of playing in any situation, which makes it a little predictable and not very adaptable. The only time we switch to a back 4 is when we're chasing a game, so there's not much pressure on the defence anyway. I think the only defender who would know his role in a back 4 is Luke Thomas (Trusty typically played either LCB in a 3 or LB in a 4 for Birmingham). I think Baldock would probably be a very good RB, but if we think the defence is a bit shaky now, how would Egan, Anel or Robbo cope in the middle with one less man than they're used to and facing world class strikers?
With that said, I don't think the problem is in defence but in midfield. At the moment, we're playing Hamer, Norwood and Souza, which is completely unbalanced. Norwood has been played in the middle and drops very very deep, meaning Souza is LCM or RCM (depending on which WB he's covering for) which I don't think is his natural position. I'm not sure any of them know exactly where they're supposed to be and it's leaving gaps when defending and not enough support when attacking.
McAtee fixes that problem, as surely he comes in for one of Souza or Norwood, meaning we have a balanced midfield again like we did with Lundstram and Fleck. So while I think coaching the team to play with 4 at the back makes complete tactical sense, I don't think we should panic and change to an unnatural formation just yet - let's see how we get on with McAtee in the team first...