It is very possible your child will grow up and be unfathomably remorseful (and possibly angry at his birth mom for the brainwashing and manipulation). There will be no way he can ever, in his own ledger, make up for this cruelty to you.
So treat him as that future person. Treat him with kindness, respectfulness, and quasi-infinite tolerance knowing full well this is just a temporary psychosis, jarring and shitty as it is.
If he doesn’t later question it and regret it, then with all due respect he is not your child as he does not have your actual presence in his life any capacity–not even that of a clinic sperm donor. You’d be making it your problem, forever flailing at windmills.
Stay strong.
Actual tips and ideas/suggestions: Do stuff with your son that’s really laid back and fun and popular. Doing so shows him that you’re cool and nonchalant and helps him to come away from the hijacked neurons and states of confusion and uncertainty. Be the cool unflappable parent that’s in the know for what’s cool and fun and awesome for kids. If you’re having trouble even getting to that rapport point, idk what to say because it hinges on the specifics of your interactions or him/you of which I’m ignorant.
I’m a mostly single father of two kids, boys, 11 and 14. From what I read here, what happened is not your fault, and you are not a bad person by any measure. Nobody is perfect, and in fact there is no one “correct” way to have handled your situation (though there are obviously infinitely many bad ways to have done so, a category which your response does not fall under).
Developmentally, age 4 is borderline for toddler and young child. Some will still be toddlers and others will have entered well into the “pre-developmental phase.” This means there will be a lot of things that they can handle, such as “no” and other boundaries; toddlers begin to test boundaries as early as age 2 (I’m not saying anything your child did was necessarily that, nor am I concerned with whatever your response might be).
As for destroying the house, this is not okay. If any children want to play in water, quite common, they don’t have to do it during every shower or bath day. Buy them a little (big) plastic basin to splash around in, have them go to a pool or water park, get an (economic) water sensory table… There are many options that are available for children to still get to do what they want–play with water–without major risky downsides like destroying your home.
In my opinion, the only potentially really difficult thing here is to talk to their mom. She’s an adult, and she needs to start acting like one instead of just everything “yes” without restrictions or prudence. Children need their parents for guidance and socialization as well as just being “providers.” Adults know better–not always but usually. If your children wanted their diet to consist 100% of candy, I presume both of you wouldn’t just go for it.
If it helps, you can write down and practice what you want to say to your family (so that you can follow the ‘script’ and not get overtaken by rage). You’re evidently eloquent enough to be able to do so. Aside from keeping it short and focused, it can help a lot to start by writing where you’re coming from and how things have felt for you. There’s a good chance they don’t know, and they should–heck, we find out new things about ourselves when we take this time to do so, to consider and refine our understandings of ourself.
Best wishes to you, and I’m very sorry for the extreme pain. That is definitely not something to be ignored or made light of by anyone who cares about you.