Michael Jackson passed away 4 years ago today -- you probably forgot ... but MJ's biggest fans definitely did not, leaving massive piles of flowers outside the King of Pop's mausoleum today.
To give you an idea, one MJ fan group -- called "One Rose for Michael Jackson" -- donated more than 10,000 roses to the grave site.
A rep for Forest Lawn cemetery tells TMZ, fans will continue to be allowed to place flowers at MJ's grave throughout the day ... and the flowers will remain on site for the rest of the week.
TMZ broke the story ... Jackson died from a fatal dose of Propofol on June 25th, 2009. His doctor is currently in jail.