There was active discussions about changing the title to "Come and See". Particularly when some poster artwork came back from China with Dennis Quaid and Ziyi Zhang sitting on a horse - a literal interpretation of the title Horsemen (2009).
The model for Dennis Quaid's character was Charles Bukowski, the hard-drinking, hard-living author. Indeed Quaid showed up on the first day of shooting with a full beard. The decision was then taken that Bukowski was a little too extreme a characterization for the lead role so it was toned down a little.
In attempting to identify the horsemen with their colors, Dennis Quaid's character makes a mistake. In Kristin's cell he determines that Garrison is Pestilence when in actuality, Garrison is Famine.
In the scene where Taylor is suspended and looking at his brother killing himself you can actually see the two hooks on either side of his head not move with the rest of his body.
In the beginning of the movie, when the Detective visits the second dead body of the girl, you can see the detective going to talk to the daughters, but in the next shot when their father comes in, the detective is standing next to the policeman again, and then returns to his original position in the next frame.
The movie is about four young adults setting forth the events of the Apocalypse, as told in the Bible. Each of the four take on the persona of one of the Four Horsemen. In that sense, yes, all four characters who are acting like the Horsemen are shown.