What would cause a women to bleed during sexual intercourse if they've had a partial hysterectomy more than 4 years ago?

There are a couple of reasons but may not have anything to do with partial hysterectomy. If it is new blood it could,be you are too dry causing irritation, or if vaginal wall was torn during foreplay caused by a rough fingernail. If old blood, brown, I would consult with your obgyn.