If the team is primarily PvE, even if you will PvP at times...

I'd go with the Priest.
Fortitude is a good buff to have, as is Shadow Resist.

You'll pretty much have every totem buff you'd like already.
A fourth shaman is an extra round-robin of several effects.
A Priest is totally new buffs, and a very strong PvE healer (Holy is also good PvP/Arena, due to Spirit of Redemption).

I like Mass Dispel a lot.
And the mobility healing they bring to any team (comparable to a Resto Druid, in that regard).

Plus you have the option to dual-spec the Priest and one Shaman.
So you could run double-heals, for really tough content or something that you're undergeared for, if there's no enrage timer effect.
Also, you can go Shadow Priest, with Replenishment and small party heals...
And have Earth Shield, Riptide, etc... from the dual-spec (Resto) shaman too.



A Blood DK, with a Priest (Mass Dispel) and 3x Elementals... will be an exceptional flag carrier.
If your dual-spec is Unholy, you can give your group the +8% spell power boost.
Frost is tempting, because if you disable IWT... you can Howling Blast from range, which will act similar to a Frost Mage giving your team additional nuking time... plus Death Coil is a decent Runic Power dump.