The band members of 5 Seconds of Summer have been together for four years as of today, Dec. 3!

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On Wednesday, the band's oldest and last member to join, Ashton Irwin, got super sentimental on Twitter over their upcoming anniversary.

The sentimental nature continued into early this morning as all the band members celebrated themselves and the fans that helped them get to this point in their careers.

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The hashtage "#4YearsOf5SOS" is even trending on Twitter with tons of celebratory tweets from the 5SOSFAM!

"Best 4 years of my entire life," 21-year-old Irwin gushed.

The drummer even shared a picture from when they just started out on his Instagram. The nostalgia is in full force--the guys look so incredibly young!

#4yearsof5sos A photo posted by A S H T O N (@ashtonirwin) on Dec 2, 2015 at 11:19pm PST

Irwin then shared a more recent snap and affectionately referred to Luke Hemmings, Michael Clifford and Calum Hood and himself as "Brothers."

 Brothers. #4yearsof5sos

A photo posted by A S H T O N (@ashtonirwin) on Dec 2, 2015 at 11:20pm PST

The band's Twitter account added to the celebration with even more photos. This time the band's hilarious nature was visible since they have been known to dress up in ridiculous costumes over the years.

"Happy 5SOS Day," Clifford beamed on his Twitter account.

Hemmings, meanwhile, got extra silly by fabricating the story of how they became a band.

"Four years ago Calum got down on one knee and and asked Ashton to join the band, or to marry him," he declared.

Lastly, Hood, is excited for "many more" years together with Hemmings, Irwin and Clifford.

Happy anniversary guys!