My 4 month old does tummy time each day, sometimes for longer periods than other days depending on his mood. He is still not lifting his head, instead he kicks a lot and tries to launch himself over the pillow. I prop his elbows underneath him but he still tends to just rest his head on the pillow. Even with me dangling toys and getting at eye level with him he doesn't seem to have an interest in lifting his head. His head control seems to be good in other situations though as we use the carrier a lot and when I sit him on my lap he moves his head to look around. Should I be concerned? Any tips?