As a couple others said, starting solids at 4 months is actually rather early. I believe 4 to 6 months is recommended. I breastfed both of mine, and the youngest didn't really eat anything until he was almost a year old. Breast milk is complete, so that was fine. If your GS is on formula, you should probably check with the doctor and get the one that's most nutritionally complete. I might add that babies should not be on cow's milk until at least one year.

nightowlrn has some good points.