Hi @Annielou, I haven't had your exact experience of having stage4(metastatic) and then a mastectomy being advised.
Mine was a smaller surgery. Perhaps this'll be helpful to fill in a little of the picture for you, maybe with stories from others here.
For me, after years of bone mets, with good sensitivity to Her2Positive treatment (same as your mets, I guess, cos you're having Herceptin and Perjeta), a couple of years ago I had a very slowly growing spot showing up on mammogram.
The core needle biopsy showed DCIS. (I think DCIS is sometimes called a Stage O breast tumour because it affects duct cells only, not other breast tissue.) I was given the usual DCIS option of No Surgery or Surgery (lumpectomy).
So, OK, Stage 4, but not out... I was happiest to have a little lumpectomy, and felt pretty lucky it was a solo tiny lump. I think DCIS can sometimes be more complex, e.g. lots of spots. (That may have given me a mastectomy decision.)
After the surgery the other information that the tumour pathology results gave us was that, as @primek has said, the receptors on the DCIS tumour cells were found to be a slightly different tumour cell receptor type to my original diagnosis.
So my oncologist and I have been able to work through, as best we can, the best drug match for tumour sensitivity, maybe why your doctor has advised tamoxifen.
Wishing you well for any biopsies and path results. I also went from the medical oncologist to the oncology breast surgeon then back to the medical oncologist for this bit. For me, fitting a lumpectomy in with my ongoing regular treatment was not as big a picture as what you and your team are considering for your best ongoing health.
Hoping you have the chance to talk through the info with as many people as you need. I pretty much wore people out with questions (it was also confusing for my family) until we could put things in perspective, yet it was worth it to feel focused on Op day.
For you now, most brilliantly, the mets have taken a back seat, and yet there's new crummy (to put it politely) stuff to work through. Hoping there may also be some pleasant things to enjoy in the next couple of weeks of Spring before you get to talk things through with the surgeon. Jenny x