The Australian actor - who plays Rachel's new New York love interest Brody - will make his debut on the Fox musical drama in a steamy shower scene.
"It's all about the whole topless thing!" Geyer told The Hollywood Reporter. "I think Glee needed something like that to shake things up because New York is supposed to be a more adult side of the show and that's what Brody brings to the table."
However, the former Terra Nova actor insisted that his character is "not a bad boy" or a "douche".
"I'm excited to dive deep into who he is," he revealed. "I don't think he's a douche; he's not that guy who is going to go after Rachel (Lea Michele) just to break her and Finn (Cory Monteith) up."
The new season of Glee will also introduce Puck's half-brother Jake (Jacob Artist), new student Marley Rose (Melissa Benoist) and ruthless cheerleader Kitty (Becca Tobin) as regular characters.
The show's fourth season will premiere September 13 on Fox. Glee will return to Sky1 in the UK in January 2013.
> Glee season 4 features Justin Bieber, Britney Spears tracks
> Glee exec on second Britney Spears episode: 'It's the funniest yet'
Watch a promo for the new season of Glee below: