We are four adults of One Family, while in HK should we buy Four 'Octopus Card' (or) One?

Oh, one Octopus Card can only be used by one person. Your four cannot share one card.
It can be used in HK on transportation, retailing stores, parking, self-service, leisure facilities and access control and other places like some hospitals and schools displaying the Octopus logo ('∞', like a number 8 on its side). :)