stage iv metastatic melanoma prognosisstage iv melanoma survivalstage iv melanoma survival ratestage 4 melanoma prognosisstage 4 metastatic melanoma prognosisprognosis for stage 4 melanoma cancerstage 4 melanoma survivalclark 4 melanoma prognosisclinical trials melanomamelanoma trialsclinical trial sitelung cancer clinical trials stage 4