I'm doing something of the same nature. I started out multi-boxing with 3 then took a long break from WoW. I started up again recently and levleed 3 hunters to 28.

Now I'm quad-boxing as of last week so I rolled a 4th hunter to join my team. I'm power-leveling him up to join my other 3 before they get too high up. Problem is... level 28 is my highest on the Magtheridon server. So I'm sticking to questing till 8, then RFC, then WC, and maybe SFK ... but I am wondering if questing won't be a better route since I don't have a high level to clear instances with ease?