Special to the Director's Cut Blog By Bill Keveney, USA TODAY The star constellation is shifting at The Voice. Usher and Shakira will join the NBC singing competition as coaches for spring's fourth season, taking the place of Christina Aguilera and Cee Lo Green, who will take that cycle off. Aguilera is releasing a new album and going on tour, while Green is developing new music and a scripted series for NBC based on his life. Current coaches Adam Levine and Blake Shelton will be back for Season 4. "We are thrilled to welcome Usher and Shakira," NBC Entertainment chairman Robert Greenblatt said in a statement. "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 Cee Lo. The participants on our show benefit from our coaches' expertise and their continued success in the music business allows for significant contributions to the contestants."...