I will chime in and agree with what has been said. I think 4 months is early. I breastfed my 3. I wanted to nurse them for a year, but my girls were biters at about 7 months and self weened. They still got most of their nutrition from formula after that, and didn't eat much food. I had to ween my son at 9 months because I found out I was pregnant again. Some people can nurse while they are pregnant, but I was not one of them.
. He got formula too, but ate other food pretty well. I think if he is gagging it might be a sign that he isn't ready for that texture yet. I would try going back to stage 1 and baby cereal. But like others have said, most of his nutrition should still be coming from breast milk or formula. As long as he is growing and going to the bathroom normally I wouldn't be too concerned.