Apocalypse and the Four Horsemen (spoilers/discussion)
Firstly, I think Apocalypse is the dark side of Xavier's psyche, possibly an alternate personality.
Why would I think this? Firstly, it has been shown that Ultimate Xavier isn't as benevolent as 616 Xavier, and has a tendency to do underhanded things, especially with his powers.
Secondly, Sinister was "hallucinating"? Doubtful... I think "Apocalypse" is a telepathic projection of Xavier, the way Xavier wants to be seen.
Thirdly, Cable has insinuated that Xavier caused a catastrophe in the immediate future and had to get him out of the picture. Cable's beef was with Xavier... but in every reality and every time period, Cable's beef is with Apocalypse.
That's why I think that Xavier, or at least a part of Xavier, is Apocalypse!
Then comes the Horsemen... I think we have been seeing them, we just haven't recognized it.
Sinister: has white skin (or at least 616 does) and carries a gun. He also had that hat and jewel. Apocalypse also sent him forth hunting mutants; he was originally supposed to be Death but Apocalypse demoted him, but he still could had been conquering in Apocalypse's name.
The gun could be the modern bow, and the hat and jewel could represent the crown. The crown could also be metaphorical, with his telepathic powers being the crown... or possibly, Apocalypse's blessings.
Deadpool: Wears red, preferred to use bladed weapons over guns, was a highly decorated war hero, and causes strife. It was also never said who turned him into a cyborg... and 616 Apocalypse, at least, likes to enhance his Horsemen.
Deadpool's philosophy that mutants should have to earn their powers isn't so different from Apocalypse's (616's and likely Ultimate's) ideology "survival of the fittest."
The order the Horsemen appear in the bible/the order they appear in the comics:
Sinister
Horse: White
Horse Represents: Victory/False Peace
Rider: Wears a crown and carries a Bow
Rider Represents: Conquest
Text: And I saw, and behold a white horse: and he that sat on him had a bow; and a crown was given unto him: and he went forth conquering, and to conquer.
Deadpool
Horse: Red
Horse Represents: Blood spilt on the battlefield
Rider: Carries a sword
Rider Represents: War/Persecution
Text: And there went out another horse that was red: and power was given to him that sat thereon to take peace from the earth, and that they should kill one another: and there was given unto him a great sword.
???
Horse: Black
Horse Represents: Desolation
Rider: Carries measuring scales
Rider Represents: Hunger/Injustice
Text: And when he had opened the third seal, I heard the third beast say, Come and see. And I beheld, and saw a black horse; and he that sat on him had a pair of scales in his hand. And I heard a voice in the midst of the four beasts say, "A measure of wheat for a penny, and three measures of barley for a penny; and see thou hurt not the oil and the wine."
???
Horse: Pale/Green
Horse Represents: Decay
Rider: Followed by hell
Rider Represents: Death
Text: And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him.
And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.