For as long as Apocalypse has been a threat, and sometimes ally, to the X-Men, he has also been one of their most fascinating characters. This is the consequence of living such a long and storied life. Part of what makes Apocalypse so engaging is that he always surrounds himself with the most powerful mutants of whatever age he finds himself in. Often times, these mutants become his vanguard and personal enforcers: the Horsemen of Apocalypse. Each mutant represented the same figures from religious texts: Pestilence, War, Famine, and Death.
Source：https://www.cbr.com/apocalypse-children-four-horsemen-names-sad-truth/
