If you need the heals regularly, I'd go with a Discipline Priest.
They have so many mitigation options and are amazing healers.

However the 4th shaman can easily dual-spec.
And be both dps and heals.
And a 4th elemental shaman (geared as a resto) is a lot more damage then any spec priest.

Shammy brings an extra grounding totem, extra healing stream, additional aoe.
Slightly weaker on mobile heals, but strong enough to heal anything.

It is a preference call.