My 4-month-old has a hard time sleeping anywhere except in his car seat. Is that very strange?

Time to fix this! He's just gotten used to sleeping there;it's time to retrain him to sleep in his crib. Put baby in bed sleepy but awake, & leave room. If he still cries after 5 minutes, return but only to tuck him in again without speaking or picking him up. Leave again and start gradually prolonging each interval before each return. It can take a few days, but your baby will learn!