Hi Felica,
In my opinion, four months is really early to start with cereal. I have been told by pediatricians that until about 6 months, babies systems are not set up to take any nutrition from anything other than breastmilk/formula. The reason Dr.s tell us to begin feeding babies cereal/fruits/vegs sooner is that many times parents WANT to feed their child, or they think it will help with sleeping longer through the night.
The question I have always asked of my Dr's is this: When your child was this age, did you do what it is you are advising me to do with my child to yours? Most of the time, the answer was no (amazingly enough!) They many times will just give you the answer they think you want. Is it going to hurt your baby to give him cereal now? No. Will he benefit from it nutritionally? Again, no.
None of my four kids had anything other than breastmilk/formula until they were past 6 months and never had any problems nutritionally. If I were you, I would stop the cereal, go back to strictly breastmilk so that your baby doesn't self wean at four months. Good luck and let us know how things are going.