Really, it should be a good old fashioned spring training pitching battle for that last spot or two. I'm guessing that Drabek and Redmond still have a couple of option years left, so they have the outside track, but for a team that is trying to win now it would be pretty dumb to roll with Happ if he looks crummy and one of them looks sharp. And if McGowan looks great in spring, then they should give him a shot. It's not like he has a track record to project from - he's basically just a piece of talented poo that they're throwing at the wall.
Assuming health and not considering options, I think everybody would prefer to see Hutch and Stroman as the #4/5 given the current depth chart, but that's just not very likely to happen in the context of the current roster.