4mnth old cries when time to poop but still doesn't poop. formula feed and been having this problem since 2 months old.

Poop time crying: Sounds like ur baby is constipated. Since he/she is already 4 month old you can try giving some free water between feedings. If it does not work you can give 1 teaspoon of mineral oil at night but not wiht the feedign. If still has problem you should see the pediatrician as some other issues may need to be considered.

Clenching?: If stools are not hard and painful for baby to pass, then i suspect baby is not relaxing the anal sphincter properly. Then it's like trying to push out the poop against a wall. Have your doc show you how to gently stimulate/dilate the sphincter if baby is in alot of distress to help the baby learn how to relax rather than clench when it needs to poop.