It's just like the other Books of the Damned. The title "Horsemen of the Apocalypse is referring the entire race of daemons, not just their four lords to the There's eight pages dedicated to the Horsemen out of about 65.
I don't think there is a ritual like the on in the Book of the Damned II for many of the reasons steeldragons describes... It barely even a refluffing and would be filler. That said, if you and your player are going to be dealing with daemons a lot, that book is worth it in spades. You can find the author, Todd Stewart, on Enworld as Shemeshka the Marauder. He's one of the best when it comes to the planes and is doubly so when it comes to Daemons and Yugoloths. There's lots of useful info on all types of deamons as well as their cults, which sounds like something that might be useful for you.
EDIT: Speak of the Daemon...