LOS ANGELES (Reuters) - R&B star Usher and Colombian singer Shakira will replace Christina Aguilera and Cee Lo Green for the fourth season next year of NBC's hit singing contest "The Voice," the network said on Monday. The changes in the celebrity judge line-up will allow Aguilera and Green to work on new projects. Aguilera is releasing a new album in November and going on tour, while Green is developing a scripted comedy series for NBC based on his life, NBC entertainment chairman Robert Greenblatt said in a statement. ...