Hand flapping at 4 months is usually just normal baby movement exploring their body, don't worry! A few of us mums noticed our babies did this around that age, it's part of how they learn to control their hands and understand cause-and-effect. You can't really "control" it, tapi just make sure dia punya tangan free to move and explore, and engage with toys or activities that keep dia interested. If it continues in a very repetitive way or you're noticing other things yang concern you, best to mention it to your paediatrician during the next check-up just to be safe.