What's the normal number of poop for a 4-month old? My breastfed baby does once a day only. I'm not sure if this is normal.

There is no exact number. Just as how not all adults poop everyday, babies too don't have a normal number. My baby used to poop everyday, but she now poops only once every 2 or 3 days. She is totally breastfed. As long as your baby is producing wet diapers, it is fine. I would be worried if my baby goes without a week without pooping. I've noticed that if I eat foods with higher fibre content, my baby poops after drinking milk. e.g. I have vegetable soup for dinner and latch her. She poops immediately after that milk session. Strange but true.