Which is the best octopus card for a four day trip to HKG.

If you will use the Express Train, I suggest you have an Express Train Travel Pass, which enables you to take one or two journeys on the train and 3-day unlimited rides on MTR. If wanting to use other public transportation, just top it up.

If you won't use the train, the Standard Octopus Card is recommended. You can use almost all public transportation with some discounts offered.

And both of them have 50HKD refundable fund.