I don't think it's worth it at level 61, and doing it with 4.

Just start questing and tagging level 62 mobs. Have the 70 ungrouped kill the mobs.
No hassle in LeaveParty and AcceptGroup macros anymore.

No worry in grabbing maintaining agro a multiple mob pulls inside instances.

If you can do that kind of levelling multi-boxing, more power to you.

I find is less stressful and more enjoyable doing repeat content since my lowbies are benefiting from it.