Fourth hole. The final one. For now; once you get into more complicated workings it's pretty common to do a couple more rounds to carve a few more holes in the world that are tuned more precisely. Maybe this book will get into those, maybe it won't - I'm still deciding how much I want to put in it and how much I want this to just be a basic primer that gets you started, and equips you with a bunch of visual metaphors that you can take to other books that are mostly just words.
You can see the whole thing at once on my website; there's even more of it on my patreon if you feel like throwing me a few bucks every month to keep me working on this, and other projects.