Just came back from my breast surgeon, who told me about BIRAD 4 that (i) for radiologists it is a kind of "better safe than sorry" category, where it might be cancerous (but probably isn't 20-30% are), and (ii) many radiologists use that category very liberally in part due to liability concerns. It is routine for BIRAD 4s to come with a recommendation for a biopsy. Once it is in writing, the primary physician then has no choice but to have the biopsy performed (again, most likely from a liability perspective or b/c perhaps their own professional malpractice insurance may require it. )
In my case, I was given BIRAD 4 becuase the growth is solid, with no secondary evidence of cancer. The breast surgeeon feels it is very likely to be fibreadenoma (sp?).
Hope that takes away a bit of anxiety for someone. It sure did for me, esp. b/c the radiologist only told me that I had a "growth" (didn't tell me it was pretty small --only 5mm) and that I needed a biopsy right away.
Anyway, I would rather be safe than sorry as well.