Aww, I get it, excited ka na mag-transition! But honestly, momsh, this is something na mas better pa rin makonsulta with your baby's pediatrician or pedia nurse, kasi they know your baby best and can check kung ready na talaga siya.
A few of my mum friends actually waited until their babies could sit up consistently on their own, pero bawat baby talaga ang pace, so don't compare! For now, siguro ang safest pa ay ang infant car seat na designed specifically para sa 4-month-olds. Your doctor lang talaga ang makakasabi kung kelan pwede na boosterseat, nak!