dang...like a little kid @ x-mas....lol
you don't need a certain number to achieve a certain tuning. you do, however, need a certain number to provide the proper port area. since your going with the aeros, i woud say two 4" should do ya.
if your box is already built, and i think that's why you said 4.9cu', then each port should be 8.75" long for a tuning of 35hz. if your using the precision port, then subtract 5" from this and cut your center tube to this length....3.75" long. this length accounts for port displacement.
however, if i misinterpeted you, and the box is not built yet, then your ports will be 8.25" long each and aging, if using PP's, cut the center tube to 5" shorter....3.25" long. this length does not account for port displacement.
as far as the sub displacement goes, it won't make athat much of a difference. if it makes you feel better, add 1/2" to the lenghts i provided.