Feedings

Just trying to get ideas how everybody does their feedings? My baby girl is 4 mons now! She eats every two hours, 6 oz a feeding and she is now flying right through her formula. Which her formula is Nuthramigen and a lot of you know how expensive that is😬. Her pediatrician recommended me giving her 1 tsp of cereal a feeding but I tried it and she got really constipated. Her poop was very hard that she bled while pooping. I’m so lost on what to do now😭. I don’t want to give her any more because of it.