As mentioned before, 4 shamans is best for PvE. Since you have to level all the way up to level 70 through PvE, I'd go for shamans.
I chose 4 shamans(no extra) because of:
- regular (out of combat) resurrection
- wipe resurrection
- healing spells
- trucks full of helpful totems (even area of effect)
- mail armor at 40 (handy stuff for surviving the more difficult instance bosses)
- you can spec one of them as a healer if need be

With 4x a level 22 shaman, you can instantly kill a level 20. Someone of my own level will take maximum 6 seconds to kill. That's enough damage for me

[edit] If I'd have 5 characters, I would go for 5 shamans because:
- one can be the main healer
- 5x same character is easier to control than 4 + 1. You'll have less macro's and keybinds to set up and maintain.