According to Wikipedia:
"The Chosen of Abaddon: Four top-ranking elite space marines within the Black Legion, and personal attendants to the Despoiler (possibly in a similar manner to his predecessor's advisors). Two are mentioned by name: Devram Korda, the Tyrant of Sarora (a servant of
Slaanesh, based on descriptions of his appearance), and Ygethmor the Deceiver, a
Tzeentchian sorcerer who led the Chaos forces in the
Battle of Medusa V. The third member is armed with a glaive, has a horned head and a snarling wolf mask, and grinning skulls hang from his ornate power armor. The fourth member is covered in scabs and sores, and continually oozes gangrenous fluid from rusted tears in his bloated power armour. These two marines, respectively, represent the Chaos gods
Khorne and
Nurgle. Also referred to in the Medusa V booklet is Zaraphiston, a fellow sorcerer of Tzeentch, who was deposed by Ygethmor. Zaraphistron has since regained his position following Ygethmor's death at Medusa V."
I think this info is probably pretty reliable, because I have heard of Ythegemor and Zaraphiston's hatred towards each other from the very Medusa V booklet the article describes and also from the 3rd Ed. codex, vying for Abaddon's favor as head sorcerer. I didn't know that Ythegemor died in Medusa V, though. So I guess it's:
Zaraphiston: Head Tzeentchian Sorcerer
Devram Korda: Slaaneshi
Khornate champion
Nurglish champion
Have fun!
EDIT: I just realised that this inner circle of four advisors is just like the Pre-Heresy "Mournival" from the Horus Heresy series: four advisors to the legion's commander. I bet you could have some fun with that.