A shaman would be decent, however I'd also think about a Resto druid, they offer some extremely fun possibilities.

The problem with 4 Hunters in PvP is goint to be the fact that you pretty much have to stand still to be effective. Close quarters is a no no. Given the excellent snaring abilities a Druid has, combined with a hunters awesome ranged DPS, you should be able to massacre any players who come within range, and hopefully kite them should they get a bit too close.

Druid offers EXCELLENT Heals over Time, great snares and decent buffs. Whilst I wouldn't go all the way Resto (due to Tree of Life not being able to cast some of the more useful Balance spells) I think it would definately be worth getting a decent way into the resto tree and also into the balance tree. :-)