Doctors say illness in her vocal chords caused singer Celine Dion to cancel four more shows after missing her Las Vegas return engagement debut last week. The Canadian singer had to skip her return Wednesday at the Colosseum in Caesars Palace at the last minute when doctors advised her not to perform because of laryngitis, the Las Vegas Sun reported Sunday. Fellow Canadian superstar Shania Twain filled in for Dion Wednesday. Dion\'s doctors have ordered her to rest for seven days to ensure a full recovery from the virus that has caused inflammation of her vocal chords. With four shows canceled, fans are hoping she can return by March 3. Dion said she was upset about the cancellations. \"I feel terrible about not being able to do these shows. I feel so bad for the fans. I don\'t like to let people down. It\'s the thing that bothers me more than anything,\" she said. \"I really hope everyone understands how sorry I am.\" Refunds will be given on all tickets for the canceled shows, the Sun reported.