You are right to be cautious. 4 cm seems pretty big. I can not imagine that being taken out by the biopsy process; 4 mm, maybe. You need a second opinion or a biopsy at another facility. In answer to your question, have I heard of this happening: only if chemo was used in advance of the surgery. I had a 1 cm tumor that was hit with chemo and I was clear before my lumpectomy. I still had radiation and arimidex. I was treated by great doctors at MD Anderson. I have been clear for 8 years now.