for pve you should definitely go with a paladin tank - low maintainence as most of your agro is caused by reflective dmg , just run into the mobs and let them damage themselves on you (this is not the 'perfect' way to tank as a pally but it will suffice and let you concentrate more on the multi tasking).

4 shamans will be slightly easier in terms of only having 2 classes and higher armor, a priest and 3 casters will give you aoe and crowd control but less survivability if you aggro.

A priest and 3 locks will mean you only have to buy the priest land mounts, and unless you have 'main' to fund your multi boxing this shouldnt be sniffed at - it's expensive.

If you are rolling alliance you will be susceptible to fear with a caster group (roll undead), which isnt as big an issue as if you want to pvp but should also be taken into account.