Stage 4 to my spine in 2021 at age 56. I'm still here, still going strong, and still in full medically-induced remission.

Everyone's experience will be different, but especially if the stage 4 cancer is "oligometastatic" (just a few metastases) and "castrate-sensitive" (responds to hormone therapy), the prognosis could potentially be good, especially if he's getting modern treatments like radiation to his prostate and metastases, and one of the second generation ARSIs (the -lutamides) as part of his hormone therapy. More here:
https://www.scientificamerican.com/article/treating-prostate-cancer-at-any-stage/