If you recruit one from each account then all 4 grouped together, if all are near each other and around the same level then you should get the 300% xp gains. If your 70 is in the group then no (even though it may still work it won't soon), if you use your 70 to kill quest mobs quick, make sure to ungroup the 70 before turning in the quest for the 300% xp. Might be faster to create yet two more dummy toons to run with initially to bring up your two new ones to 21 instead of boosting with the 70 since you can get 300% xp with just the 4 lowbies.