Our pedi gave us the go ahead at 4m Because she was already taking interest in our food we chose oatmeal over rice cereal. I mixed mine with breastmilk back then now at 11m we just add water.

Always check with your pediatrician first... BUT it is fine to start at 4 months. If you are breastfeeding, mix the cereal with your breastmilk for a very runny consistency. Babies are used to pushing their tongues out to get milk from the breast, so it is a whole new technique to scoop food in with their tongue. You will also want to get video the first time you feed your LO cereal bc it is funny watching them figure out how to keep the cereal in their mouth.