@Alice Third year. Decades go 1-10, so it's 2021-2030. This is from beginning of modern period of AD when calendars went from 1 BC to 1 AD. There was no year 0 (zero). This means decades are 1-10, not 0-9.
@Alice was saying it’s the fourth year of the year 2020 (Not the fourth year of the decade). I’ve heard jokes are funnier when explained. Hope that helps.