If you are gonna stick with quad boxing, and not go the full 5, I'd go with either a paladin or a druid. Dual spec means that you have either a tank or a healer, and you can pug a 5th, of just about anything. If you pug a healer, go tank on the pally/druid and switch pets to the DPS ones. If you pug a tank, switch to holy/resto, and use dps pets. If you find another DPS, you can either try to power through it with 1 tank 4 dps with dps pets, or use tank pets, and heal with your pally/druid.

It would be a complicated setup, but you would be extremely versatile, and wouldn't need to lvl up more toons