I know this question is from three months ago, but thought I'd add my experience. I was diagnosed with stage 4 prostate cancer last October. There was metastasis to the spine, ribs, lymph nodes, and pelvic area. Interesting enough, I've had very little pain.
My treatment is pretty much what I call first line standard of treatment care. I did six cycles of chemo (docetaxel) and started hormone therapy with Lupron shots (ADT/Androgen Depravation Therapy) at the same time. My initial PSA was over 5,000. I'm done with chemo but still on Lupron, and most likely will be for life. But docetaxel + Lupron was very effective. My PSA had lowered to 22 last checkup, which is elevated, but a far cry from 5,000. As long as I remain hormone naive, then I'll just keep getting a Lupron shot every three months. Androgen suppression is a big part of first line treatment. (Note - generally if you've got metastatic prostate cancer, a prostatectomy is not really an option, as systemic treatment is what you'll need.)
Once you become castrate resistant, then you'll likely need to look at second line treatments. There are a lot of options, and this seems to be the area where a lot of clinical trials are for metastatic prostate cancer.
I am not a doctor, only a patient, and my experience has only been with hormone naive metastatic prostate cancer. And since I am not a doctor, I can't give you any number for life expectancy. There are so many variables. I will say I've met guys 10+ years past their stage 4 diagnosis. Best of luck to you.