If you have 4h, you realistically can spend 1-2h either downtown or in Parque das Nações, which is much closer to get some fresh air and have a coffee. As long as you arrive an hour before your flight to Faro, you'll be fine. It takes 30 min to get to downtown, 10 min to get to Oriente. If you don't encounter any delays on arrival, I would consider the trip.