Reddit reviews Wet-Stop3 Green Bedwetting Enuresis Alarm with Sound and Vibration Bed Monitor For Bedwetters: Potty Training Children and Kids: Bed Wetting Alarm For Boys or Girls Proven Solutions For Nocturnal Enuresis Alarm System, Curing Bedwetting For Over 35 Years

I'm sorry your MIL is so awful. My siblings and I all had problems with bed wetting growing up. I don't know where you might find one, but there's and alarm system you can buy to help him train himself to wake up at night. It snaps to his underwear and it goes off when it gets wet. It worked like a miracle for myself and my siblings. https://www.amazon.com/Wet-Stop3-Bedwetting-Vibration-Bedwetters-Solutions/dp/B0013LRVWA/ref=sr_1_1_a_it?ie=UTF8&qid=1504365688&sr=8-1&keywords=bed%2Bwetting%2Balarm%2Bfor%2Bkids&th=1
Edit: found it on amazon, and highly recommend it

Bedwetting alarm?

I was a bed wetter until 10. The only thing that worked was an alarm that attached to my panties (I wore 2 pair) and a box that would sound and vibrate to wake me up. It took a week and my body learned to wake up when I had the urge to go.

It was like this...and based on reviews it might work!

http://www.amazon.com/gp/aw/d/B0013LRVWA/ref=redir_mdp_mobile

https://bedwettingstore.com/dry-me-bedwetting-alarm.html

Just the one that clips outside the underwear. It was cheaper on Amazon, but it's not in stock there now. They have systems where the alarms goes in a pocket in special underwear, but they are a bit pricier.

Edit: Just found this. Looks very similar.
https://www.amazon.com/Wet-Stop3-Bedwetting-Enuresis-Vibration-Options/dp/B0013LRVWA?crid=2NJYQU4YLAF2E&keywords=dry+me+alarm&qid=1536452195&sprefix=dry+me+al&sr=8-2&ref=mp_s_a_1_2

They make alarms for kids that clip to your underwear. Something like this. Depending on how hard you are to wake up, this could be helpful. I'm sure diapers would work, but that feels like giving in to the piss to me.

First off, when starting, be sure to have extra pajamas and sheets nearby, to make changing them easier. Another possibility is to just lay a felt blanket over the area and not change the sheets til morning. The felt will keep your son dry. Preferably there's some waterproof mattress protector.

Then, you've noticed he pees multiple times a night and you may have an idea as when he tends to pee. One solution is to set an alarm clock to wake you up, so you can escort him to the bathroom before he tends to pee (eg 10pm and 2am). As he's 4, you might then be able to set the alarm in his room and have him become independent in this regard, but I would do it with him while he's learning.

In addition, there are bedwetting alarms that'll go off once he starts peeing. Some parents find the alarm useful in helping their children become more aware of their bodily needs at night as they get immediate feedback as to when they're peeing rather than a delayed response. This then allows them to respond appropriately, either by holding it til morning or getting up and using the restroom.

Here's an example:
http://www.amazon.com/Wet-Stop3-Bedwetting-Enuresis-Alarm-Vibration/dp/B0013LRVWA#

Wetness alarms have existed since at least the early 1980's, and are still around today. No need to design from scratch, unless you're have a very particular or special design goal.

Malem is one such device, along with a similar product from Rodger, and there's another from Wet-Stop. These products generally clip into the underwear or onto a waterproof bed pad, and are intended to wake the wearer after they've begun to wet. The idea is that they wake up and associate the need to finish urinating with the action of waking.