Stages have to do with tumour size and cancer spreading. They usually use them TMN system for prostate cancer:
https://en.wikipedia.org/wiki/TNM_staging_system
I have stage 4b (distant metastasis), but there's a new way of considering things, where "oligometastatic" cancer ("oligo" means "few") is more-and-more often treated with curative intent.

Traditionally, with stage 4 they'd tell you to expect to live about n months or years and treat you palliatively to keep you as comfortable as possible until the end.

These days, if you're strong enough and your onco team is up to date on recent research, they'll "throw the kitchen sink" (my team head's actual words) at stage 4 oligometastatic prostate cancer. There are people here in the forum who've gone over 10 years at stage 4 without serious progression.

Long story short, the stages don't always mean what they used to. Even with stage 4 PC, don't stop saving for retirement just yet. 🙂