At my wits end with my 4yo

As the title says, I'm at the end of my rope. Also, I have long nails that I only get once in a full moon and am on my phone. So pls excuse format and errors, I'll try and fix the ones I catch. I'm just gonna let it out, if it makes sense great! If not I'm sorry. I am 9 weeks post partum.. saying it like that feels like such a short period of time. But living it, it feels like it's been an eternity. Any way, I have done everything I can to make my son(4) feel included. Have time where all 3 of us play together, 1 on 1 time, tell baby brother to wait when big brother needs me, let him help when it's things I feel are within his capabilities and baby will still be safe. It's apparently still not enough. I don't know if it's 4y territory, or new family dynamic but he has been acting crazier than normal. Extra screaming both good and bad, extra crying episodes, and extra sensitivity. He also doesn't take me serious. Im attempting gentle parenting, NOT permissive but true gentle parenting. And unless I snap and start shouting like Lous from MITM he won't listen. He'll, half the time he's LAUGHING at me. I've also tried redirecting some behaviors, explaining why somethings just aren't gonna fly and none of it is working. Unless I whip out my chancla and ACTUALLY land one on him he won't listen. He "pleads for his life" is what imma call it, in a mocking way other wise. I have taken the ps4 away and same situation. If I snap I scare him, understandably so, and he starts crying. I have tried breathing, walking away, counting to 10, and he just keeps pushing through all of my "defenses" until I do. I don't know what to do anymore. I don't want to rely on the chancla to get him to listen every time. I see it as I'm raising a future adult and he needs to learn how to act in the wild without me, plus I always say "what exactly is hitting him gonna solve?". Idk if i need advice or just to vent but yea here this is.