Shakira and Usher are to replace Christina Aguilera and CeeLo Green as mentors for Season 4 of the U.S. competition series "The Voice," NBC announced. Current "Voice" coaches Adam Levine and Blake Shelton will stay with the show when it returns for its fourth outing next spring. Aguilera and Green, who are leaving the series to fulfill other professional commitments, are expected to return the following season, Robert Greenblatt, chairman of NBC Entertainment, said in a statement Monday. "We are thrilled to welcome Usher and Shakira to 'The Voice.' The addition of these popular iconic artists continues the tradition established by Christina, CeeLo, Adam and Blake," said Greenblatt. "Doing two back-to-back cycles of this show requires a tremendous amount of time, and we are happy to accommodate the commitments made by Christina and CeeLo. The participants on our show benefit from our coaches' expertise and their continued success in the music business allows for significant new contributions to the contestants when they return."