Mom's who went from Breastfeeding to Formula

When did you move from breastfeeding to formula? We just went in to our 4 month appointment and my lol has gone from being in the 40% for weight to the 9% I feed on demand and if she hasn't eaten in a while, I feed her anyway. She's never fussy after eating and she quits taking the breast after she's full. I understand where my pediatrician is coming from some she dropped so far in the percentile. He's suggesting to top off with extra breast milk or formula. I totally get why but I hate the idea of feeding her and then feeding her again, expecially since she doesn't seem hungry.