Had robotic hysterectomy & had sex @4 weeks. Now having a bloody discharge. Not a lot but enough where I can tell what it is. Whats could this be?

Vaginal cuff. The vaginal cuff is an area where the walls of the vagina were sutured closed after the cervix was removed at your surgery. New granulation tissue is "sealing the seam' and may have been disrupted during sex. Call you gyn surgeon and go in to be examined to see if that is the case. Sometimes this area can be cauterized in office with a silver nitrate stick.
Unsure. The safe thing to do would be to see your surgeon for an exam of the top of the vagina. The bleeding after 4 weeks is not normal and could be the sign of a serious problem.