Concerned About Mini-Push-ups

My LO turned four months today, born 1/23/17. She has very good head control when I hold her or lay her on my chest for tummy time. However, when we do tummy time on other surfaces, she will hold her head up just briefly and then just lay down. I've tried engaging her with songs, toys, etc...but she just isn't interested. Because of this, she still doesn't hold herself up on her forearms (mini push-ups) and I'm concerned that she should be doing this by now. 😔 She has her 4 month checkup next week, and I'm going to ask her ped, but I am interested in hearing your experiences. Is your baby doing this already? If so, how old were they when they started?