Booster Seat

My fiancé switched our sons car seat to the booster seat and I absolutely hate it. The thought of my son riding around in the booster seat makes my heart race because it just doesn’t look safe at all. He’s still so small that his neck barely clears the shoulder strap and it’s uncomfortable so he pushes it behind his back so all he has in a lap belt basically. And I mentioned that to my fiancé but he got defensive and shut me down. I’ve tried having a conversation with him about it but it’s his way or the high way. And I’m pissed because it’s so selfish. Like this is about our son’s safety. Idc if our pediatrician said we could - doesn’t mean we have too or should. I’d prefer to keep both of our kids in car seats until they’re literally too big for them. But I can’t talk any sense into my fiancé. My son is 4 btw.

Is 4 too young to have him in a booster? Am I being a helicopter mom by wanting to keep him in his car seat?