Famine: Omega Red. Because of his embarising loss to Spider-Man, Omega Red would be a complete and utter failure in his mind. He would even contemplate suicide. Apocalypse would come to Omega Red and give him an offer to become a true force of power and might. Apocalypse would extremely enhance Omega Red's death spores to the point where they would destroy organic life on a mass scale, mainly vegetation and the such. Omega Red would gladly take this offer as a Horseman.
War: Proteus. While Betsy Braddock survived Colossus crushing her to death, David Xavier's subconsious would survive as well. He wasn't as experineced as Betsy with his psychic abilities so he would not find another comatose person to jump into. Proteus's mind would be drifting in the Astral Plane as he grew stronger and watched the X-Men. In Cry Wolf while Jean and Xavier were serching for Rogue and Gambit, Jean heard a voice tell he "Jean... turn your attention to 350 5th Avenue" Jean asks the professor if he said this, but he ignores it and continues on with the search. The voice could have been David speaking out for the first time since his "death". In the Ultimate universe, Apocalypse could be a telepath of the likes of Xavier and Jean Grey, and would have heard this message as well. He would have been watching the X-Men as they had recently defeated Sinister in Tempest. Apocalypse would invite Proteus to join his Horsemen with Xavier as a barganing chip for David. While David would be the most powerful of the Horsemen and completly subservient to Apocalypse, he wouldn't have as much power as he used to but would have more control. He would bring war to the world for Apocalypse so he could have his father to himself.
These would be four logical choices for Apocalypse if writen this way. IT would bring improvments to two severly underused characters (Multiple Man and Omega Red), bring back one of the greatest X-Men Beast, and pull one of the X-Men's most powerful members back from the brink.