You need to step back and realise that it’s not healthy to be so obsessed by your ex and all the details of his relationships.
It’s got absolutely NOTHING to do with you.
Why are you raking up details from almost 5 yrs ago and putting yourself in the middle of it?
Clearly you haven’t got over your relationship breaking down. But it was a very long time ago now and you need to make your peace with it and stop taking over the details and living in the drama of it all.
Work out what’s relevant to your child NOW, and if there’s anything you need to do to secure his wellbeing.
That’s in terms of making sure he has a continued relationship with his sibling, absolutely not inserting yourself into the middle of your ex and his partners relationship breakdown!
I get that it’s brought back an upsetting time from the past for you, but really, this thread is saying a lot more about you than about them. I really don’t understand what you ‘being postnatal’ (?) has got to do with a relationship breaking down four years later?!
Oh and if you are genuinely thinking that there might be a correlation between the child’s ages and the father walking away... maybe it’s the age when the child is becoming an independent person, and not a cute accessory? Children start to require more of a relationship than the ‘patting on the head’ easy type of relationship some people get away with acting like to babies and toddlers? Treating them more like pets than real people? But at 4yrs, children react more to the negative stuff as well as being grateful for a pat on the head?
Complete random guess though, as 4yrs isn’t exactly a known age for becoming so unappealing that fathers walk away.
I don’t really believe there’s anything in it here either, I think you’re just determined to find a way to put your own stuff into your ex and his partners split.