4x shaman is the best in arena. Anti-fear management is key in being succesful at high rated. (Staggered tremor totems). Winning still doesn't come easy though, to do arena succesfully you first need to gear yourself up, and such. So before you'll get high rated, there's quite some grinding to be had. Fact is tho, it has the potential unlike no other multiboxing set up.)

4x shaman is one of the more fun classes to hop around in bg's with. Still having good anti-fear management and deadly burst. Still, in bg's you can get away with any number of mages/hunters/warlocks (with a priest healer probably pref then.) Anything really.

4x shaman is very capable of heroics, even Kara. Heroics are considerably easy in fact when you have a good geared tank tanking for you. Even Kara is a possibility, I've done a full Kara run in one night on my four shaman (Was a PuG, one night was impressive!). I've yet to do any other pve content as a quad-boxing shaman, so can't say about anything else.

So, you looking to be able to do all content? Definatly 4x shaman. I realize like 90% of the boxers are elemental shaman. But heh, it has a reason. xD And it's a -lot- of fun!