At 4.5 months, some hand flapping can be a normal part of how babies explore their movements and express excitement, but since it's happening very frequently and he's hitting himself, it's worth bringing up with your PD just to get a proper assessment. The fact that he can still control his hands to grab things is a good sign, but your PD can look at the full picture and tell you whether it fits within normal development or if there's anything to watch.