4 month old & pooping

my little girl is 4 months (preemie) she was on similac sensitive and wasn’t pooping every day. I ended up buying her the walmart “similarity” since it had the exact ingredients and because it’s SO much cheaper. and after a while she would poop a hard ball which is from constipation. I went to her pedi and she suggested giving her less than 1oz of water or to switch to soy. I tried the water and nothing, now shes on soy..still poops the the same but now she’s pooping everyday. (which is good)

anyone else experience this? she didn’t advice prune juice or anything i’m assuming cause she’s a preemie?