my almost 4 month old LOVES laying on her back. She's the happiest then and plays/talks the most. Tummy time is a nightmare and she lasts a few minutes before she freaks out (even when we lay her on top of boppy pillow). We do also have her sit up in our laps, but that's not something we can do all day as you can imagine. I worry about flat spots because of it or not progressing as fast because of rare tummy time. Any advice?