Figure at least 40 minutes to get off your flight, clear customs get out of the airport and to any form of transportation. And you want to be back in the airport an hour before your flight. So that leaves you 2.5 hours rather than 4 hours.
In theory you could take a train downtown, walk around for 25 minutes, and take a train back, but that probably wouldn't be too satisfying.
Note that the int'l terminal is T5, you'll have to take the shuttle train to another terminal to catch the train into the city. You can catch a taxi outside T5, lines usually are short, and go to a nearby restaurant. Sweet Willie is the expert here on restaurants in the ORD vicinity. I know there is a Korean one that he's currently big on