I can confirm that once you reach that stage, 3 and 4 are great and very speedy.

I used between 1 and 4 characters to boost my 5th shammy to 70 and it worked pretty well.

1) The usual boosting and with a few key quests to 40.
2) Boosting with only 1 shammy (out of group with lowbie tagging 1st still gives slightly more XP than grouped) up until around L55. This is the point around which you can no longer 1 shot stuff.
3) Progressively add one more shammy to the boost group as you level so that you can still one shot stuff. For me this meant 2 boosting (again out of group from 55-62, then 3 for 63).
4) Post 63 (or it might even be at L63) your points 3 and 4 kick in and you can let your full force rip on everything

I did my last level superfast by just grinding on the elite demons in Nagrand with the 4 shammies as backup.

Enjoy