stage iv metastatic melanoma prognosisstage iv colon cancerstage ivstage iv melanoma survival ratestage 4 melanoma prognosisstage 4 cancerstage 4 metastatic melanoma prognosisprognosis for stage 4 melanoma cancerstage 4 rectal cancerstage 4 esophageal cancerstage 4 metastatic cancerstage 4 pancreatic cancer