After somes researches and tests, I’ve got a formula that works.
IF({Date 1}=BLANK(),IF({Date 2}=BLANK(),IF({Date 3}=BLANK(),IF({Date 4}=BLANK()," ",DATETIME_FORMAT({Date 4},'YYYY')),DATETIME_FORMAT({Date 3},'YYYY')),DATETIME_FORMAT({Date 2},'YYYY')),DATETIME_FORMAT({Date 1},'YYYY'))
Thank you !
Quentin