“The Four Horsemen of Atheism” is first and foremost, a marketing term. The term was coined almost exactly a decade ago, in 2007, in order for the horsemen to sell recordings of themselves. From there, the term had runaway success.
It appears that the reason that Dawkins, Dennett, Harris, and Hitchens were chosen (instead of other well-known atheists) is that they were all best-selling authors of atheist books in 2007. It also arose from media coverage, such as the famous 2006 Wired article, which coined the term “New Atheists”, and interviewed Dawkins, Dennett, and Harris. (Hitchens hadn’t published his book until 2007.)
But for me, it was never the books which were important, it was the blogs. I started reading Pharyngula in late 2006. I only ever read one of the books, and I read it in 2008 and didn’t care for it. To me, it has always seemed odd how much we venerate book authors. There are other media outside of books, after all! What about bloggers, journalists, youtubers, podcasters, and artists? Or for that matter, any more recent authors?