I have personally never seen, nor needed to call, simultaneous stalling for the fourth time in a match. That surely be an interesting situation, but I think unlikely in the grand scheme of things This is probably why this specific of a case is not covered. I guess if I was pressed with this situation, I would just call the stalling penalty on one of the wrestlers, which in my judgement was stalling "the most".
I hope that we wouldn't get to that point in a match though. It would have to be both wrestlers just laying there on the mat or in a clinch/tie up, 4 times, simultaneously. Proactive officiating with verbal instructions during the course of the match, and good/effective coaching, would likely prevent this situation from occurring in the first place.