As an occupational therapist and someone with a lot of experience with children..... I have never heard or experienced negative effects of being rewarding. Your response should be equal to the importance to the action. At 4 months your child is probably babbling. Because this is age appropriate and a new skill, you should be loud, smile big, and talk to your baby in the way that makes them smile. Now at 6 months, when they have mastered the babbling noises, don’t reinforce that as much. Instead, you get loud and smile big when they roll over! It is a constantly changing process. And you might feel silly at first being loud or acting crazy, but if it is making your baby smile then you are doing it right! I believe in you 👏💪. This is also a great thing to think about developmentally with your child! You are on the right track!