Its hard to figure out a 4th spec when most classes cover everythign with just 3. So a 4th could simply be a spec beyond the 3. Not really a "jack of all trades" spec but like a mastery spec that goes beyond the specs or a step farther into a single spec... (so naturally some of these might overlap while I think about this)
warrior - blade master or gladiator (a sword n board dps class)
death knight - Lich... UH and FROST are melee with spells, well this would be the flip to that, obviously.
paladin - Fallen pally or oath breaker. Spriest with plate and a 2h weapon. Get reck'd. probably the easiest spec to make out of all of these imo.
hunter - i dont have a name for the spec, but the hunter would become a savage with the blood of their animal running through them. almost like a pseudo-feral druid. Melee spec
shaman - a witch doctor or something where they could summon elementals and buff them (or players) and debuff anything/anyone... so, uhh *cough* a warlock with nature spells?
monk - idk... it have to be something that changes the class. Like a "Dragon knight" (ie ancient chinese soldier but with dragon animations) or [insert Jet Lee movie roll here]
druid - maybe a caster ability where they harness the aspects as buffs or attacks. "but thats balanced" you say. No... they would not be limited to arcane, nature, and the moon... this would harness bear and cat spirits AND nature itself.
rogue - ranger or a ninja. as a ranger they would obviously use a bow, but as a ninja their "Daggers only" abilities would expand to swords and/or demon blades.
D-hunter - idk
mage - Sage, wizard, something superior to their self. A mage is only a master of one spell class. A sorc/wiz/archmage would be beyond that harnessing all elements and the universe. Not that OP... they'd just be flashier than normal and have new gimmicks.
warlock - Im thinking demonology... but absorbing the demon to transform. Each demon is different with differnt rolls, therefore "demonized lock" would be too. kinda like a druid with all their forms.
priest - inquisitor or zealot or something that is opposite of spriest and is all about CC and holy dmg