Is it normal for a 4 month old BB to be peeing lesser n not poo for a day despite usual milk volume intake?

I do not quite understand about peeing less often. Do you mean freqency of change is lesser with light diaper or the frequency of change is lesser but heavier diaper? I wouldn't worry if it's the latter as baby grows, their bladder grows. The brain will trigger bladder to release the muscle when the threshold amount is reached. Pee and poo lesser could also mean that baby is absorbing the milk well such that the amount of milk feeds might be insufficient. I would latch more frequently till the frequency of wet diapers increases. As for poo, it's normal not to poo everyday. I will be very much concern and bring baby to see he doctor if baby doesn't poo more than 7 days. In between I will do tummy massage to stimulate poo-ing.