I dont know how much bigger the cells are, but if they are not much bigger than the 4200 very carefully file out the plastic strap holder and the little plastic bump you can see in my pic to help shoe horn them in, to give it that little bit of extra width.
Personally I hate using tape, I have seen too many peoples races come to a quick end when it snaps, not only that the 4600s are a little heavier so when they are jolted suddenly the tape will snap.
Try doubling up the tape and see how you get on, but imho its the size of the 4600 cells that is giving you the problem. You swapped out the velcro holder for tape because the cells wouldnt fit, but the tape is a weaker replacement.
I am sure with a dremmel you could make those cells fit nice with a bit of time and still use the velcro holder....