Yes mine does it all the time like folds her body over. It's hard to put her in her car seat and Down sometimes. She enjoys doing it lol. But it doesn't seem to do her any harm.
Ours likes to arch his back when he's lying down.. I think it's his attempt at rolling over/getting a better look without rolling over. He's also formed a bad habit of throwing himself backwards when he's being held.
I actually asked our doctor about this at the four month appointment. First she looked at me like a crazy FTM for asking if arching the back was OK. Then she said - "Why would you think it was a problem? It's not at all." I guess you CAN ask your doctor too many dumb questions! Haha.
Re: 4 month old arching back