I would say it would be close to impossible. Landing at JFK then waiting for luggage, then getting taxi to nyc I would think is easily close to 2 hours. Getting from NYC to Newark could take as little as 45 minutes but don't count on it. Realistically 1-1/2 hours then you need to be at airport 2 hours before flight leaves because security will be so long and slow. Leaving maybe 1 hour to look around? Not worth the aggrevation.