Four up, ominously, as the end approaches? (8)
I believe the answer is:
horsemen
'approaches?' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.
I cannot understand how the remainder of the clue works.
(Other definitions for horsemen that I've seen before include "Riders" , "knights, perhaps" , "The Bible has Four . . . . . . . . of the Apocalypse" .)