4 month old outgrowing car seat?

My baby is super long. He head is at the top and feet all the way at the bottom. I was thinking about getting a convertible car seat for him. The kind that can decline and rear face. But I’m not sure if I should wait till 6 months. That’s when it seems like most people switch but he just seems like he is out growing it! Also I can’t take the insert out because there isn’t another head rest and there isn’t any cushion under it for him to sit on. Just wanted some other opinions since I’m a ftm.