Breast biopsy report said that they took 4 biopsies but pathology report said "3 cores measuring 8-9cm each". why aren't there 4 cores?

It may be lost: It is possible to have lost one of the biopsy cores during processing in pathology. Or maybe the pathologist assigned to your case simply made a mistake and wrote 3 instead of 4 cores. In any case, it is worth contacting the doctor managing your care or directly the pathology department at the institution the biopsy was made and asking them to investigate and find the reason for the discrepancy.

Good question: You need to ask the doctor who ordered the bx done, as you probably can't speak to the radiologist.

Good question: What matters is that all of the tissue taken in the biopsy was submitted to the pathology lab and that it was all processed for histology. It's pretty common, especially for a long biopsy core to fragment into smaller pieces.