Ah Dr. Google. We all do it. Most people say we shouldn't, but that doesn't stop us! Now I'm not a medic but googling fibroadenoma looks good to me! First, it's common, and second it's benign. Hooray! Benign is the best word in the world here on this site! Your phyllodes tumor theory looks unlikely to me because this tumour is very rare. OK, they are removing it. Probably because 4cm is on the larger side. But I honestly don't see why you should be making yourself ill with worry. Try to relax, trust the medics, and keep telling yourself that the lump is benign, because this is precisely what you have been told by the experts. You'll be fine. Best wishes. Harry