The Lisbon metro is so so easy and very cheap. Make sure you have a card that has chip/pin access to use the metro ticket machines. Just buy enough for 2 fares (each), something like 3 euros. Very easy to get in and out of the city from the airport stop. The ANA lounge is OK, but I certainly wouldn't want to spend 6 hours there. Are you flying to the US? Plan to be back at the security checkpoint with boarding passes ready about 1.5-2 hours ahead of time to get through security and then immigration, as well as gate security.
Go into town, get a coffee and a pastry and take a stroll enjoying beautiful Lisboa.