the most popular 5 box team is pali+4 shammies... so I would go with 4 shammies
shammies= totem buffs everywhere, everyone can heal if needed, main spec dps, then offspec one as a healer for hard bosses
once ur gear gets good enough (at level cap), u can 4 dps through the instances, but when ur gearing up u might want one as a healer
but leveling just make them all dps (elemental spec), to dps and just put up a heal when needed