My 4 month old baby has not passed poo for 4 days now he is on breast milk and formular, is that normal?

NORMAL: Breast fed babies can have bowel movement from 6-8 times/day to once every 4-7 days. Both normal. If ur child is not uncomfortable, don't worry. Formula may be changed to 'whey protein' based, e.g. Good start. Whey protein increases stool bulk, so s/he will poo more often. Again, if the baby is not uncomfortable, not to worry, no change needed. Talk to baby's doc reg starting solid food.