One thing to consider (although you probably already have)... It might be easier leveling up two groups of two than trying to do all 4 at the same time. Some of the instances throw in a ridiculous amount of adds when you have three or more in the instance. Easiest way around it is to keep all four toons in your group, but only complete the instance with your main and one of the alts.

I just did an instance for the "General Durant" Sith Warrior quest, and with a full group, it added two strongs and an elite to every pull. Like I said, ridiculous.