Mine was/is a 4 oz. kid too. He would usually take 24 hrs. in a 24 hour period. Sometimes he'd get a little extra in there, but 24 was his average. All babies are different. Hopefully the new formula and the upped Zantac doseage will help your little girl out and get her gaining a little better.
Extended rear-facing, non-circ'ing mom to 3 rotten boys - P (9), J (almost 8)), and E (2.5)