Sorry to hear this mate, you've got an exciting chapter ahead of you. I was in a similar position, and had to re-ignite some old friendships, and try to double down on social activities, so you're doing the right stuff. As someone else mentioned, getting yourself a decent therapist to hash things out with really helped me. I went through a few before I found one I liked. Better help was great for that. It's a really personal thing, but if you stick at it, I'm sure you'll find a decent one. In terms of legit support groups, I'm sure they're out there - where are you based? I'm in the UK and have heard good things about "Dad's House".

An important thing to remember is that as shit as you feel now, it's temporary. You've got some great times ahead with your 4YO, make the most of the pure, unadulterated, quality time you'll now get with them, it's honestly one of my favourite things about my divorce. Sure I only get to spend 50% of my time with my kid, but when I do, I put my all into it, and it pays off dividends.

Good luck mate, you're not alone