Puree/Solids at 4months?

Hi moms! My lo is currently 4mo premature so shes a little underweight for her age. Pedia told us ok naman si baby but if we want her to gain more weight we can start solids na if we want to! A little nervous! Medjo low supply ako ngyn sa breastmilk so we’re thinking if it’s best to introduce solids/puree na OR formula milk? Hope you can share your experience! TIA 😊