To get most out of multiboxing experience you will need to level 2 teams if you want to pvp and pve. 4 Mages are super fun and fast early on but once you get to endgame dungeons and want to gear up you need a proper tank for some bosses and for other bosses you can kill it is far more risky and frustrating without a one, one voidwalker does nothing when you have a mage dps.

Voidwalker is a good tank when you have a team of warlocks who can slowly dot all targets and spread damage taken between between all 4 of them, so you can clear any 5 man dungeon with them and clear UBRS at least up to Rend if you have proper coordination and skill.

If you want portals and soulstones you can level a team of warlocks to 20 and park them where needed, this way you will be super useful to your raid group.

If you want to do dungeons alone you need a tank to clear all content, if you don't want a tank I'd go with 4 mages and run high level dungeons with a tank, which will not be hard since he can get all the tanking and melee dps drops, and runs will be smooth since you will have only 2 people coordinating pulls