What actually makes stage 4 colon cancer terminal ? is it organ failure ?

Varies widely with: the individual yet correct. if fatal, 1 of more organ system functions become so compromised that our vehicle/body becomes ↑ingly fragile & any number of otherwise easily handled variables result in death. Additionally, with awareness of ↑ing debility & sing options, often pain with very advanced disease, the internal will to live fades & dying becomes the better option. See: https://goo.gl/JgoS6z

Not always fatal: Distant mets characterizes stage IV colon cancer. In many instances there is a solitary lesion to lung or liver. These can be resected and in selected instances, chemoembolization to liver or hyperthermic peritoneal chemotherapy used for control of distant sites. With introduction of chemo-immunotherapy a higher degree of cure can be anticipated in not too distant future.