I believe it's a better idea that X-men staff will give us permanent members of the four horsemen of Apocalypse. So who do you want them to be? They don't really need to represent War, Death, Pestilence and Famine. Besides, the real four horsemen are called Conquest, War, Famine and Death.
After going through the process/rituals to officially make them a horsemen (my idea is these four horsemen have accepted to become loyal horsemen of Apocalypse. All can breathe and survive in space too.
Kenji - powers enhanced, immortality, high telepathic ability and can grow as large as the Asian continent.
Chimera - powers enhanced, and one thing I want to emphasize is Chimera can generate gigantic thousands of flare dragons who have minds of their own. Invulnerability, superstrength and can move as fast as the Helios probes.
Garokk - powers enhanced, and one thing I want to emphasize is he can create thousands of huge independent stone creatures.
Kierrok - can summon thousands of ngarais with different powers and abilities. Kierrok now has intangibility too.