lol. Sorry. I had to. My then 4 y/o grandson repeatedly peeded in the glass and nickel trash can beside the toilet in my guest bathroom (and missed)so frequently that the metallic coating came off of the metal. He was very potty trained, but had fairly bad aim. That isn't something a grandmother can help with by the time they are that old. As the guest bathroom was only used occasionally, and I'd clean it once a week, I couldn't figure out how the urine got in the can. I lectured him repeatedly on having better aim and complained how I was tired of having to clean the area after he used it. Finally, I realized that I should just remove the trash can every time he came over and that solved the problem. But I only figured out what he was doing after he forgot to dump it in the toilet one time. Oh, and there wasn't any other child or adult with opportunity at the correct times.
So, yes, at least one boy at 4 y/o is capable of some "pee-fest". When I told my other daughter who has two sons who were 3 and 8 at the time about it she laughed and said that she too had just had her boys doing a pee fest into the bathtub at about the same time, and she lived in another state far away.
Thus, it's most likely that one boy started and all 3 joined in, and the 3 y/o may have had help getting his diaper back on. It would be just like that age to not realize that it would eventually be discovered, and just like a 6 y/o to instinctively say he knew nothing as he is well aware of where his urine is supposed to go by that age.
I'd tell the other mom, but I'd do it in a laughing at the funny/horror story sort of way, admit to not watching the boys closely enough -- although it's possible it was done in less than 10 min. as my grandson would only be in the bathroom that long -- but more likely longer because of the variety of locations -- and promise that for sure next time you'll keep a closer eye on them, and she should just be aware that they seem to have a new game which needs to be nipped in the bud or she's likely to find an unappealing surprise herself.
Likely, with that sort of approach she'll be apologizing to you for her boys part in it, promise to tell her boys to never do that again, and pay close attention when she next babysits.
She could however, go off the deep end and blame you for not supervising minutely, accuse you of lying, insist your son was either the ringleader or sole participant, or just think you absolutely disgusting for even telling her about it. If she does any of the above, I'd politely tell her that she's on the wrong track and you do hope that someday her head can find it's way back to the correct one, but you'll not be at the station waiting for that to happen.
Good luck!
Oh, and I recall my 3 brothers having distance contests even when they were a few years older than these boys. Yes, they were horribly embarrassed when I made my presence in the back yard known. This is truly one time I can say boys will be boys and there wasn't anything 'mental' about any of it.
'Course, if it happens again, I'd say there was some mental or degenerate thing happening to at least one of the boys and a serious discussion should happen with the other parents so you can get to the bottom of it.