At this point, I'll believe it when the cameras are officially rolling. Sly and the producers really have themselves to blame. The first one was pretty much on par with what the entire gimmick of the film should have been -- the old action guys from yesterday teaming up and blowing s*@t up. As I stated on the podcast, I think that EX2 put in a little too much humor, but the film still worked. By EX3 though, the series became a full blown parody of itself. Which is fine if that's what the intent of the series was supposed to be... which it wasn't.
Dolph's gotta pay the bills, and even with the stellar year he's going to have thanks to CREED II and AQUAMAN, I can't imagine that he'd say no, even for a cameo (which in the end is pretty much what the roles in these films become), but at this point, I'd say it's done. Sly should have really stuck to basics and not threw in all the needless "young guys" who lended no personality or style to the film.