My daughter weighed 13 pounds at her four month check up & her doctor said it was safe to start cereal, primarily because she spits up a lot and we struggle to get her to drink her bottles. It's hard for me to get her to finish 20 ounces in a day and, as the last post said, it's more important for them to get milk than cereal. I mix mostly formula, with a little cereal, simply to get my daughter to consume more formula.
Have you tried adding an ounce to each of your daughter's bottles? That might help her to last longer between bottles. I've also read that you should transition in 15 minute increments. So, if she has bottles at 7 AM, 10 AM, 1 PM, 4 PM, and 7 PM.... they would switch to 7:00, 10:15, 1:15, 4:15, and 7:15..... I think the final "tank up" bottle can remain at 7:00, since your daughter's bedtime won't change when switching to the 4 hour EASY. And then, once she handles this change well, you'd move to 10:30, 1:30, and so on....up to four hours between bottles.
Congratulations on getting your little one to sleep through the night! My daughter still wakes up at 3 or 4 AM.