I think they also keep 4 QBs. Dobbs is far too talented to think you can stash on your practice squad. They can't cut Rudolph because they just drafted him. They would look like idiots if they did that. Landry Jones is a veteran who has played a couple of regular season games so knowing how conservative NFL teams are, its unlikely given Big Ben's injury history that the Steelers would go into the season with 2 guys at backup QB that have never played in a regular season NFL game.
So you're right. They will be forced to keep 4 QBs as long as Dobbs looks like he did last night. Which is a guy oozing with talent.
If Dobbs plays well through the entire preseason, I actually would cut Jones and go with the two young guys.