We will visit in September on a cruise ship. We will probably have about 4 hours to visit. I believe a shuttle will take us to the Comercio area. Will we have enough time to visit the Tower of Belem and Monument of Discoveries? If so, how best and quickest to get there and return? Or would we be better to simply walk the downtown area? Only 4 hours and we do not want to miss the ship. The time will be 12:30-16:30. Thank you for any info/advice.