It would probably be best if you can hold off altogether on the solids. 4 months is really young, and such tiny, underdeveloped tummies aren't really ready to process solid foods at that age.
My son showed several signs of readiness around 5.5 months so we gave solids a try for a couple days. It irritated his tummy and it was very clear that while his brain might have been ready, his poor little tummy was not. We stopped solids completely, and he was much more comfortable. He wasn't truly ready until closer to 7 months. I think that the AAP 6 month minimum age recommendation is pretty sound.
At least be aware that solids in general (not just the constipating ones - we started with pear and it was problematic because he was simply too young) can cause pooping or other digestive issues in such a young baby.