The Four Horsemen are described in the Book of Revelation (the last in the Bible). They appear when Jesus breaks the seals on a scroll given to him by God. The implication is that the coming of the Horsemen will mark the start of an apocalyptic period for the world.
As with many things in the Bible the topic has been subject to much debate and various interpretations. I would take the view that they are neither agents nor enemies but an allegorical instrument used to describe events yet to come.