I don't personally multibox yet, but here's my two cents.

If it's not too much of a hassle to keep the 5th, I'd definitely keep it. You're most likely gonna level up to 80, and it'll be more fun with 5. And if the only inconvenience about it is that you have to swap with 2 in 5v5 arenas, then I hardly see it as a reason for abandoning it. There WILL be times when you want to have 5 of them, you can be sure of that. :P