Hi, I have plan to stay in HK for 4 days, should I buy Octopus Card or AE Travel Pass for 3 days?

They are different. You are advised to buy the AE Travel Pass. You can use it to ride on AE train, plus 3 consecutive days of unlimited travel on MTR (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories). It is very convenient for you.

By the way, the 3 days unlimited ride starts from the recorded entry time of first MTR ride, and lasts for 72 hours. On the fourth day, you can recharge some money on the card. Then it can still be used on the public transportation.
But don't worry. The AE trip it still available on the 4th day.