Is it safe for me to have breast reduction surgery while living with stage 4 MBC?

If your medical doctor clears you for elective surgery, you might be a candidate for this. Go visit with your primary care physician first.

Did your MBC have any primary tumor in the breast that required surgical excision, eg lumpectomy, followed by radiation?
If yes - breast reduction after radiation is always a bit tricky as the breast tissue and skin doesn't behave normally and can settle in unpredictable ways. I recommend finding a plastic surgeon who also specializes in oncoplastic breast reconstruction. Many plastic surgeons will be uncomfortable operating on a radiated breast.
If no - it depends on how much weight you've gained. I generally recommend that patients are at BMI less than 30 to have best surgical outcomes, especially if operating on a radiated breast.
In either case, your oncologic medications may suppress your immune system and make it unsafe to proceed with surgery. Coordination will be required between you, your oncologist, and your plastic surgeon.