I will be in Hong Kong for 4 days with to and from the airport on the first and 4th day.

You are advised to buy Airport Express Travel Pass with one/two single journeys for HKD 220/300. Also, you can travel on MTR (except Airport Express, East Rail Line First Class, Lo Wu and Lok Ma Chau stations) free of charge within 72 hours after swiping it at the entrance gate for the first time. After 72 hours, you can top it up, and continue to use the card as a normal Octopus card.