Introduction
I am going to start off by saying this is a crackpot theory, and to be honest, it's a less of a theory, and more of an observation. This is something I have noticed, going back and looking at all 6 X-Men films, as well as the upcoming Days of Future Past. This observation will likely not have anything to do with future X-Men films, and it is just something of a pipe dream.
I'd also like to briefly bring everyone up to speed on the Four Horsemen. I'm sure most of you already know of them, but for those who don't, lets sum it up. Good ol' Apocalypse is a very dastardly villain, and in my opinion it isn't just because of his ridiculous power level. One of the parts of Apocalypse that I find the most interesting are his four horsemen: Famine, Pestilence, War, and Death. Over the years we have seen neumerous X-Men take on variations of these four horsemen, and rarely is it... consensual. With Death we have seen, tons of X-Men, most notably: Angel, Wolverine, and Gambit. We have seen Sunfire as Famine, MR. Sinister as pestilence, and the Hulk as war. I won't be focusing on the X-Men as horsemen very much until the end, so just stay with me.
Famine
I'm going to start with Famine, simply because of all the connections I have made and will make, Famine is easily the weakest link. Besides that, between you and me, I think Famine is the least cool of all four horsemen. Famine represents starvation and when I think of famine, I tend to think about barren crops and dried rivers. Well which X-Men character, in the films, does anything like that, or has any similar motivation to that? I'll tell you it isn't, porcupine head.
No, who I am thinking of is far more devious than a man who can hug you to death. It's actually pretty ironic, given that his last name is a food, and no, I am not talking about Brian Cox. I'm talking about Sebastian Shaw, played by Kevin Bologna Bacon. Sebastian Shaw expiremented with Mutant potential, and unlocking a mutant's full portential for power when he was a Nazi, but perhaps more notably he is the reason Magento is what he is today, that'll be important later. "How does Sebastian Shaw in any way resemble Famine?" you ask. Well it's based in his power sets, he can absorb (kinetic) energy, for all intents and purposes he can pull the life out of something, causing it to wither and die, it's a weak connecton I know, but it's still there. Let's go beyond that, let's look at Shaw's big picture plan. He wanted to spark nuclear war between Russia and America, because in this future nuclear wasteland the mutants could rise up and take the world as their own (that alone sounds very Apocalypse like). So if you are willing to reach just a little bit, then you could say Sebastian Shaw is the Horseman of Famine.
Pestilence
Now this one is a bit more obvious. Pestilence is illness, sickness and poisons. People catching the plauge and falling over, green faced with that look of bewilderment on their face. I think of lots of greens and chemicals when I think of pestilence. Now how on the world, could this be? Who is the one character featured in an X-Men movie who frequently uses poisonous chemicals, but also uses green as their motif color? OH, I KNOW, I KNOW! IT'S ROUGE, I mean she makes people sick, and she wears that green cape, it's rogue, right?
No, true believers, it's not rogue (I mean wouldn't she be famine anyway?). Not it's that other mutant who wears green, I'm talking about Madame Hydra Viper. Viper is first and foremost, a scientist, who specialize in mutant powers, specifcally transfering mutant powers from one living being to another (geez, I wonder why Apocalypse would be interested in this). Her powers are based on poison and illness, being able to produce paniful toxins, and then cure them shortly after. She was even able to inhbit Wolverine's powers (I hope Trask Industries payed her royalties for that inhibitor collar). Viper didn't really have a big endgame, she just seemed to be doing work for the pay, while at the same time pushing forward with her own personal ventures. So with little to no reaching, Viper is quite obviously the Horseman of Pestilence.
War
Okay, if you can't guess this one right now, then I have lost all faith. This is the one that made come up with this crackpot theory in the first place. War is exactly what it sounds like, murdering one another, going to battle, and by far most importantly building weapons. This guy is not only deeply imbedded in the government and military, but he is aslo notorious for his rather...creative weapons. He might not be a mutant, but he is still one of the X-Men's greatest foes, that's right I'm talking about Col. Hendry.
Okay, it might not be that unimportant character who was featured strangely prominently in First Class. I'm talking about William Stryker. I'd like to say that I really like Stryker, he was great in X2, and one of the not horrible parts of Origins. He is something of scientist who likes to play with mind control, as well as pooling mutant powers together into one big weapon XI (that also sounds like a skill Apocalypse would like in his crew). Stryker is also the reason for Wolverine being as powerful as he is. I mean there isn't much to say here, he is the embodiement of war, from his time in the war to his work destroying the X-Men. He built the "ultimate" mutant weapon...twice. I think it's fairly obvious the William Stryker, while not a mutant, is the horseman of War.
Death
Here we go, the final, and the biggest one. We all know Death. Death must embodiement death and destruction, none can escape it and all must submit to it, Death is what the other Horsemen lead to, Death is the end. When Death takes over, the Apocalypse has begun. Who, in the Fox-verse, could embody death adn every bit of it? Well I'm going to give somewhat of a copout answer. They all are given the color motif of black, and their leader is a feirce, powerful person, who hates one side of the mutan-human war. I think we can all figure out that I mean Calisto and the Omegas.
HAHAHAHa! Can you imagine if I was actually serious this time? All that build up, and to what? to one of the lamest parts of Last Stand, I mean just look at that fella on the left, boy? girl? I don't know, and honestly I don't care.
Who I am actually refering to is Bolivar Trask and the Sentinels. He is a scientist who predominantly focuses on new way to screw over and kill mutants. He is responible for the machines that wipe out countless mutants, but also humans, they are the harbingers of the end times, quite literally. They are unstoppable forces of destruction, mayhem, and of course death. What's his endgame? to kill all mutants, how does he go about this? By kiling all mutants. If you need anymore proof that Trask and the Sentinels are Death, just look at the Mark X, it looks like the grim fudging reaper. I can say with confidence that Trask is the Horseman of Death, and his sentinels are the harbingers of the end times.
The Theory Itself
Apocalypse's right hand man, for the most part, is Mr. Sinister, a mutant scientist who is an expert of transfering powers, pooling them together, killing mutants, and bringin the world to a desolate wastelend. Each of my proposed Horsemen are scientists in some regard and all deal with mutans in even more regard. They are all pretty evil and represent their aspect fairly well. The theory, which has a 1% chance of coming to fruition, is that Mr. Sinister used these four indivuals to not only make himself and Apocalypse stronger, but to help usher in an age that Apocalypse could easliy conquer the world in. Using the research of Stryker and Viper, Sinister can pool hundreds of powers into Apocalypse. With the ideals and weaposn of Shaw and Trask, he has created a world, that fears mutants, a world where mutants will rise up against their "oppressors."
It is a pretty loose theory, but it's just something I have been thinking about for a while. Feel free to berate me for some of the more dumb parts of this theory, or you could contribute and tell me who you would place in these roles.