II know there are lots of folks who dispute the idea of very large enemas, but if someone has megacolon, wouldn't he or she NEED very large enemas to get completely clean? I have absolutely no idea what the limit is, but I think it's at least a worthwhile question to ask.
As far as the enema entering the small intestine, through the cecum, there is actually a reference in medical literature, which I will try to find and quote, but the gist of it is that a 19th century doc used a huge enema delivered at very high pressure to eliminate (reduce?) an intussusception. the patient was a 12 year old girl, and she was positioned head down with the bag above her bottom 5? 8? 10? feet above her bottom. The water was very hot, 130 degrees, and forced through her colon until she vomited from the water flowing through. It was probably horrible for the girl, but the intussusception was eliminated and she recovered. I would guess she didn't want another enema very soon, though…the nonsense about what can and can't happen probably needs to be taken with a grain of salt.
I have been continually surprised by the things that people say don't exist, or happen, that do happen. I suspend disbelief in lots of cases, simply because I've seen scientific denials disproven so many times by events.
As far as danger, it's certainly an issue, and everyone should be careful. It's like climbing a ladder. I can still do it, but it is not advisable because my balance is sometimes compromised. I don't do that.
As far as 4qt. enemas, I used to take them occasionally--two drugstore bags consecutively with no expulsion in between. I found them wonderfully cleansing. When I had a colonoscopy, the doc found a diverticulum, and I don't fill myself that full any longer. I recently did 3 qts., but that's probably my current limit. I definitely don't want a “blowout.” I wouldn't be surprised if 4 qts. or more was within my capacity, but I probably won't try. If I had a knowledgeable female giver, maybe, but not by myself.