Hi Amanda,
I am not aware of any nutrition related research in Stage IV breast cancer specifically. However, based on the research in early stage and breast cancer prevention I would be very supportive of your vegetarian/vegan diet.
I thought you might enjoy this link to Diana Dyer's website. She is an RD and breast cancer survivor. She follows a vegan diet and is a wealth of information on the subject.
http://www.cancerrd.com/
Best wishes,