Therein lies the rub.
Whether you're 1:1 or 4:1, unless the guide literally has eyes in the back of his head, a guide at the head of the line cannot possibly see anyone behind him/her. So that means they have to turn around to confirm everyone's there. How often should that be? Every 30 seconds? Every 10 seconds? Pick any time and you can still argue that a diver can get away unnoticed.
So hypothetically (JUST TO BE CLEAR, I'M NOT ADVOCATING THIS AS A REAL SOLUTION) let's put the guide at the BACK of the line so he/she can see everyone ahead of him/her. But now you've got a guest, possibly untrained in cave/cavern dives, leading the way. So that won't work.
Absent 1:1 side-by-side with the guideline in between them, there's not a good solution here. I know the few times I've done cenotes dives with one of my groups, I always take the rear position so the guide can lead and I can keep an eye on anyone who strays.
And maybe that's the answer. Require two guides for every uncertified group and have one at the front leading and one at the back watching for trouble.
Unfortunately the answer when Mexico is in the topic is rules are bent and pushed to the limits in pursuit of profits. It's the way it always is. Cheerleaders can get upset about it, but there are plenty of examples and any of us with any decent amount of experience in Mexico all have the stories to prove it. There is no safe way for a cave guide to guide more than one non-cave certified divers in a cenote dive this death just proves it.