In my opinion she is too young for solids. I'd quite them totally until she is at LEAST six months old, can sit unassisted and does not have a tongue thrust reaction when feeding her solids. The recommendation these days is what I just said, not 4mos. We started our oldest at 4mos and he did have some problems that I believe were caused by the early introduction to solids. Some old school doctors still say 4mos, but they're not up to date.
(just read the other responses ... please do not give your baby karo syrup. if she's getting constipated, her body is saying she isn't ready for the solids. don't push it. also, do not water down her formula.)