What does stage 4 colon cancer mean?

It has spread. Stage 4 colon cancer involves spread to distant organs; commonly it spreads to the liver, lungs, bones, abdomen cavity. Though these patients can have long-term survival for years in most cases it is incurable.
One more thing. I would just add to the prior answer that the average survival is about 2 and a half years--but note this is an average.