4 Greater Demons ? Sounds like Khorne, Slaanesh, Nurgle and Tzeench
I had a similar idea, giving them item drops that are unobtainable otherwise, but I did not really get to that.
There were supposed to be seven, but two of them are waiting for new features. Right now the only ones that work are wrath, famine, and pestilence. The unfinished ones are ruin (in progress), infidelity, cowardice, and temptation (in progress).
DONE
Hellion = flying humanoid creature, razor sharp claws, attacks in huge swarms.
Greater Demon of Wrath = enormous creature, enormous horns, brass skin, brutal melee fighter.
Greater Demon of Pestilence = enormous creature, toxic gas and blood.
Greater Demon of Famine = enormous creature, one arm, one gigantic pincer claw, injects venom that destroys the guts.
IN PROGRESS
Greater Demon of Ruin = enormous creature, flying, has dragon fire and another breath attack that causes all sorts of FUN.
Greater Demon of Temptation = enormous creature, ungodly natural dodge skill and resilience. Weak but irritatingly hard to kill, its main threat comes from luring military units out of position and into other demons.
WAITING
Greater Demon of Infidelity = enormous creature, it is itself weak in comparison to other demons, but has a breath attack that causes attackers to turn on each other.
Greater Demon of Cowardice = enormous creature, powerful melee fighter, has a breath attack that adds [FLEEQUICK] token to enemies.
I am 99% sure that both of these creatures in waiting will be possible with the next DF release.