A female high school student and a male character in his 20s will join the guitar-playing sibling of Mark Salling's character for upcoming episodes of the Fox series.
They are all described as being "as easy on the ears as they will be on the eyes", according to TV Line.
Glee creator Ryan Murphy recently confirmed that all of the show's regulars will feature in at least part of season four, which will split its time between the remaining students at McKinley High and New York, where Rachel (Lea Michele) is enrolled at NYADA.
Murphy also said that Glee's fourth season will revive the "underdog" theme of its early episodes in a bid to impress viewers.
"That will appease people, maybe," he suggested. "Sometimes I feel that you can't win. It's just a volatile group of people that watch it, and for that, I like their passion."
The second season of The Glee Project, in which contestants compete for a role in the show, premiered yesterday (June 5) on Oxygen.