When four strange ponies arrive in Ponyville, it doesn't take long for Twilight to notice that they match the descriptions of ancient harbingers of the apocalypse. Sure, their generally meek attitudes seem a far cry from the four dreaded destroyers, but between their amnesia and the circumstances of their arrival, Twilight finds herself faced with three possibilities:
1: This is all some big misunderstanding.
2: They are the four horses of the apocalypse, but somehow arrived early and won't be causing any trouble... right?
3: The end really is nigh, run for your feeble lives, etc.

Needless to say, Twilight really hopes it's not option #3.