C.O.
The big question is, is she peeing through the same amount of diapers? Is she gaining weight? If not, take her to the doctor. Hold off on the cereal unless the doc says to go for it. Even 4 months is a bit young. It isn't uncommon for children to change their feeding habit as they grow, because their needs change.
You may also consider changing formulas, maybe this one causes her discomfort...something to talk to the pediatrician about.
Good luck, keep us posted!