Crossword Genius

Four up, ominously, as the end approaches? (8)

Ross

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" .)

I've seen this clue in The Guardian.
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