Unfortunately, 4 hours is a bit tight for trekking into the city. By the time you get downtown, it will be time to turn around.

On the upside, ICN is probably one of the best 5 airports in the world. Aside from great shopping and dining options, if I'm not mistaken, there is also a golf course incorporated into the airport complex. The Asiana lounge at the airport is also a terrific place to pass the time.