layover
4 hrs is probably not enough to go into the city and do anything meaningful. The "L" to downtown from the airport would take about 40 minutes each way. Allowing time to get back through security on your return would leave you with about 2 hours in the city at most. A cab back and forth may be a bit quicker but would run about $35-40 each way (and you would be returning at the tail end of rush hour).