That's not how it generally works out in practice - at least, for the Cleric. For the Cleric to match the Fighter in combat effectiveness, he needs to either use divine metamagic (which is generally agreed to be very, very cheesy) to precast his buffs, or to spend rounds of combat casting (which, considering combats are usually very short, is a big problem).
All the Druid needs to do, on the other hand, is spend all his time Wildshaped with Natural Spell, and use his OOC telepathy whenever he wants to communicate anything to the party.
As for the optimal party, I'll go with a Wizard for encounter resolution, a Druid for stealth, divine casting and melee combat, a Warlock for UMD and endurance scenarios, and a Factotum for everything else (including arcane spells the Wizard can not or does not take).
Outside of blasting and Turn Undead, there's actually not much redundancy provided everyone coordinates a bit on spell and ability choices.
Edit: And they can have a Samurai as a 5'th man.