I'm trying not to over-hype it, but there WILL be a book with a sick amount of detail on the Beatles tape machines. History, use, mods, photos inside and out. Better to wait for it than to speculate on some things that may seem incorrect in the future? We are hoping for late Dec./mid Jan on the book. Very close now.
BTW - I believe you have seen the 4-track in photos, just not clearly. Or you haven't seen enough photos... it's there (sometimes).