According to this site a new horseman is coming:
http://tvline.com/2013/12/17/homeland-season-4-spoilers-carrie-quinn-romance/
Who could it be? My guess is agent Morales.
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Most likely War. Famine rode last in Ichabod's dream and I believe that revealed the order of the horsemen riding. Definitely War, been confirmed for Season Finale.
(Edited by The 5th Horseman)