If we have four solid on our side but they know there isn't a fifth, "our" four may very well not want it to come up yet.
That, frankly, is what concerns me. One of the majority in Heller
may have reluctantly voted to allow guns in the home, but has indicated an unwillingness to allow guns in public. OTOH, if the Heller
minority were sure of a fifth vote, why wouldn't they vote to grant cert to one of the cases so they could effectively restrict the right to one's residence?
I think the simplest explanation is probably correct: They are very busy and aren't willing to grant cert until a circuit split forces them to.