Meghan Trainor Named Final Judge on FOX's 'The Four'

Meghan Trainor will fill the remaining seat on FOX's new singing competition "The Four" ... TMZ has learned.

Production sources say FOX execs were noodling on 3 female candidates for the final spot -- Trainor, Fergie and BeBe Rexha. She'll be joined by some superstar talent on the panel ... DJ Khaled, Diddy and music honcho Charlie Walk.

The show -- which has an emphasis on hip-hop -- features 4 contestants who will face challengers throughout the course of the season who try and knock them off their spots. The winner will start a music career mentored by "The Four" judges.

We're told Meghan will actually tour with the winner and help launch the singer's career.

The show is set to premiere in January.