As long as the the 4 toons that are leveling, are somehow recruited to each other, it makes no difference. I had an 80 priest take me through DM, Stocks, SM, and Strat. No exploiting, ie leaving/joining party, just a fast run, aoe, nuke through.

Depending on MY lvl, I would get 3/4 to 2 full Levels per run, this includes, an 80 running me through Deadmines.

Example, at lvl 10 you would get 2 lvls a run, through DM. As you level, it wil go down, but it is way faster then questing, by a long shot.

Edit*
I got to 60 in less than 2 days played, and that was longer than it should've taken, because I did not always have someone to run me through.