The 4 and 5 spots are addition by subtraction, however, this is inconsequential to the case in point here:
Playoffs.
In the playoffs, they need three starters. That would place Aceves and Bard back, making the pen a force to be reckoned with. The top three for the Sox is phenomenal and so would be the playoffs bullpen.
I understand that you simply cannot be objective on the subject and continue to hang your hat on "last year's ERA", but that really doesn't matter when assessing potential playoff pitching.