Your visa is valid for four years from the date of grant.
Your visa letter will tell you the last possible date to validate your visa (= travel to Australia once to activate it). Since police clearance certificates and medicals have to be valid at the time of first entry and are only valid for a year, that date is usually 12 months from the earliest issue date on medicals/PCCs. You can return to your home country after the validation trip or move right-away.
Note that the 4 years validity starts ticking down immediately after the grant date. To quote from the 489 visa page: