Prostate cancer is one of the more less risky type of cancers and a person having stage 4 at his age means there were sighs of this disease years before getting to stage 4 and would have been treated prior to getting to this stage. Its not common at this age and just asking are they sure about this and maybe getting a second opinion.
I am very familar with prostate cancer as i myself am a survivor with 45 radiation treatments which stopped it in its tracks.
First are you sure its stage 4 as is uncommon esp at his age.
Every case is different but the prognosis doesnt sound too good here. 5 years maybe depending on therapy regime and overall health. It could be an exception case though so dont take this info for granted