There's not really any specific number by the time they get to that age. It's more about what their normal habits are, and noticing if there are any major changes. They should probably go at least once a day, but missing a day isn't necessarily a reason to panic. It will also really depend on if you are breast feeding or bottle feeding. As long as there are wet diapers, it's ok if there aren't poopy ones every single day. Best thing to do would be to call your pediatrician if you have any concerns.
A 4 month old has around 1 to 2 poopy diapers a day. At this stage, the baby does not usually poop a lot since they are mostly on breast milk or formula. Its best to monitor the number of times the baby poops so as to notice any problems.