Got some questions 'bout general Multiboxing.

[Ualaa]

You can box four accounts on one PC fairly easily.
Warcraft is not a terribly demanding game.

I'd recommend an i5 or i7 based system.
A Q6600 was fine, the previous expansion but mass battlegrounds like Tol Barad and SW/Org might not be the smoothest on a similar system.
It's a good idea to have 1GB of ram for the OS, plus 1GB per client; a 64-bit OS lets you exceed 3.2GB of ram.
Wow is not very video dependent, it is mostly CPU.
A cheap SSD is a decent upgrade for many, but certainly not required to box.

Four shammies, druids or warlocks are all decent 4x teams.

If you want to run PvE, consider going five accounts.
As that gets you Tank + Heals + 3x DPS.
Not saying you have to have all five, but it is a nice option.

For PvP, especially the smaller BG's, having only four of ten or fifteen is a strong option.
The larger BG's won't make a huge difference between four or five; you'll blow people up fine either way.

For something like Death & Decay, Hurricane, Rain of Fire, etc...
You would want the option to send next click from your software.
Put them all on the same hotkey, and have it set to broadcast your mouse until you click.
So you can "aim" them all at once.

Other spells without ground targeting is much easier.
You need an assist method.
Focus works, as does Target or Party Leader.
If you're doing non-arena, Focus is the easiest.
FTL is a stronger assist, which is a little complicated to set up, but strong once you do.
Some software will do FTL for you, I believe Game Commander Pro and IS Boxer.
FTL is an assist based on adding a modifier to your assist key, so the slaves/other windows know where the assist call was issued from.

If you know the name of the ability... for example:
/cast [pet:imp] Firebolt
You can macro it.
/cast [@FocusTarget,Pet:Imp] Firebolt

Master/Slave are general terms.
You can switch from one to another, depending on your software this can be instant to a couple of seconds.
Some software will make the new toon the leader automatically.
Others you'll need to push a macro.
Once you do, the new toon is the leader/master, and the others are the slaves assisting/following the new lead.
Jamba is an addon that is highly recommended for boxing and uses master/slave stuff.

I might try to have one Druid cast Entrapping Root, while another does Solar Beam over the target via mouse broadcasting.
You could sync up the camera angles, and then do a spread out macro.
Which would then hit a larger area, than a single cast.

I'd take a look at the software forum.
My personal recommendation is IS Boxer.
There is a 7-day trial period, and Lax gives phenomenal support.
If you're comfortable writing scripts, take a look at Hot Key Net.

[Kekkerer]

4 or 5 Moonkins for arena is a terrible idea, you only have one cooldown (trinket) to get rid of aoe fear. I've got a 5 druid team that I use strictly for pve, they are a nightmare to use in battlegrounds and probably much worse in arena.