An enhancement shammy adds a lot more then the other two, unless you want Mortal Strike/Wound Poison quite badly.

The DK is there for the melee buffs, the Death Grip and the snare effects. Basically, to cover the weakness of a lack of ranged dps, and the lack of an ability to stop them from kiting you.

Each pally has an aura and a blessing for the team. Probably: Devotion, Retribution, and Resist type aura's. A 4th pally is two resistance aura's, while only 3 is one less aura. Then we go with Sanctuary, Might, Wisdom and most likely King's. The single target version of Sanctuary is Str/Sta boosts, but the group version is only Sta; if the group version is fixed then that's basically an almost useless 4th blessing.

Each pally brings Divine Storm, which auto heals the most damaged toon in the party (kind of how Chain Heal jumps to secondary targets) and an extra Art of War proc (which is either a self heal vs AoE/Cleave or a directed heal to the most hurt toon) or DPS. Losing the 4th pally, is the loss of Divine Storm/Art of War. Also lose your 4th Divine Sacrifice etc.

But then you add Warrior (Mortal Strike, Shout) or a Rogue (Wound Poison/Crippling Poison/+Dmg misdirect move) or an Enhancement Shammy (Bloodlust/Heroism, Heal Stream/Mana Spring, Tremor/Strength of Earth, Windfury Haste, Fire Elemental?). Not really sure if Windfury stacks with the 20% haste from Frost, or if Strength of Earth/Warrior Shout stack with Horn of Winter.

Anyway, balance the loss of an aura/blessing/art of war proc, vs whatever you gain. A warrior wouldn't be that more squishy then a pally, but a rogue or shammy is a much squishier toon.