my child 4.5months old, he keeps every time mouth opening and if we make a hand gestures near he will be more active and opens mouth widely and shakes hands and legs

A. That’s probably out of excitement a lot of kids Breathe from their mouth in this is so maybe your child is doing the same which is a normal thing if you find anything odd in the behaviour then you can get it examined with the Dr but with what I understand there is no problem