I think it'll be closer to 600' by the time you circle around and enter the ground floor "lobby" toward the upper-left of the image, under the "rotunda." (Using the measure distance feature on the map)

In T4, if the flight departs the A concourse, it will be ~900-1,300' from the T4 Airtrain; if it departs the B concourse (occasionally VS flights do, not sure about the morning departures though), its about 900-3,000', yes, potentially more than a half mile! Actually, add about 200-250' for the walk to the escalators up from the air train and then to checkin.