Evil comes … #ElementalEvil pic.twitter.com/m8ImOBwnaA

— Christopher Perkins (@ChrisPerkinsDnD) February 19, 2015

… in many forms. #ElementalEvil pic.twitter.com/fBzjEiSTJu

— Christopher Perkins (@ChrisPerkinsDnD) February 19, 2015

The King of Dungeon Masters, Chris Perkins, gives a blast from the past, something from an ancient tome called Fiend Folio: here are described the four Elemental Princes of Evil that will be in the upcoming campaign adventure Princes of the Apocalypse.

Master Chris is giving suggest me another thought : Princes of the Apocalypse adventure narrates about 4 Elemental Evil Prophets that are linked with their Princes of Evil, here their connection:

• AERISI KALINOTH, the Air Prophet ⇒ Yan-C-Bin (Prince of Evil Aerial Creatures)
• MARLOS URNRAYLE, the Earth Prophet ⇒ Ogrémoch (Prince of Evil Earth Creatures)
• VANIFER, the Fire Prophet ⇒ lmix (Prince of Evil Fire Creatures)
• GAR SHATTERKEEL, the Water Prophet ⇒ Olhydra (Princessof Evil Water Creatures)

If you want to dig in the knowledge about The Four Elemental Princes of Evil you need to search their alias: The Archomental.
In Planescape settings there’s the Planescape Monstrous Compendium Appendix III that well describes all of them and their relationship with the Tharizdun/The Dark God/Elder Elemental God, they are powerful but not demigod, they grant spell to their followers and yes, they can be slain.

If you want to play now a good prequel of Prince of the Apocalypse you can play Egg of the Phoenix adventure module where player characters must recover a powerful artifact and save an entire land from destruction by the Princes of the Elemental Evil.

Other references about four Princes are

• Dragon Magazine #285  article with new Elemental Evil monsters, how to fight against Elemental Evil and new EE spells
• Return to the Temple of Elemental Evil adventure module that is an immense adventure for charcter 4-14 level
• The beginning: The Temple of Elemental Evil adventure module.

There’s only an unanswered question:  where’s the fifth prince? The real son of the Elder Elemental God, Cryonax, Prince of Evil Cold Creatures???