Have to concur with the above. For a 4 hour layover I wouldn't bother. It's still likely to take you 30+ minutes to get to downtown and you're going to have to pass through Security when you come back to take your connecting flight, which may involve a lengthy queue if you can't use Fast Track.

LIS is not the greatest of airports to spend time in especially if you don't have status and/or can't get into a lounge but frankly with a 4 hour window it's not going to be that painful and, IMHO, there's absolutely no point in going into the city for the hour or so you could ever spend there. Come back another time for a proper visit.