In stage 4 colon cancer if appetite is poor / gone down, does this mean it's terminal?

In most progressive. advanced disease states, apatite eventually decreases, lots of individual variables. In any state of health, enough decrease in nutritional intake makes it increasingly difficult to impossible to maintain body functions.
Wasting syndrome. Many cancers cause appetite supression and an altered metobolism leading to weight loss. This is not limited to late stage disease, but certainly does impact the resiliency with which a person can endure added treatments and maintain stamina to continue to live. Some treatments also supress the appetite or cause ulcers/pain that reduces desire and taste. Do the best to get calories in to sustain,