The fourth season of American Idol premiered on January 18, 2005, and continued until May 25, 2005. It was hosted by Ryan Seacrest. Randy Jackson, Paula Abdul, and Simon Cowell also returned as judges. Carrie Underwood won the season with approximately 500 million votes cast in the season and 37 million for the finale, while Bo Bice was the runner-up. Underwood has since gone on to become an eight-time Grammy-winning country megastar. It was also the first season of the series to be aired in high definition.

This was the first season where guest judges were invited to participate in the auditions.

• Mark McGrath of Sugar Ray, Washington, D.C. auditions
• Gene Simmons of Kiss, New Orleans auditions
• Kenny Loggins, Las Vegas auditions
• LL Cool J, Cleveland auditions
• Brandy, San Francisco auditions

Where there were four judges present, three "yes" votes were required for the audition to proceed to Hollywood.

This was also the first season where the age limit was raised to 28. Among those who benefited from this new rule were Constantine Maroulis, Bo Bice, Scott Savol, and Nadia Turner.

Auditions for the fourth season began on August 4, 2004, in Cleveland, Ohio.^[1] An audition was originally planned for Anchorage, Alaska, but that was cancelled due to a possible schedule conflict as a result of hurricanes affecting the Orlando auditions.^[2]

There were 193 contestants in the Hollywood rounds. They were split into two groups and performed on separate days at the Orpheum Theatre in Los Angeles. The contestants first chose from a list of twelve songs selected by the producers. They were separated into groups of males and females, but they performed solo with piano accompaniment and back-up singers. 95 contestants were eliminated after this round. The remaining 97 contestants competed in the second round which featured group performances. The songs for the groups were Four Tops's "I Can't Help Myself (Sugar Pie Honey Bunch)," The Temptations' "Get Ready," Barry Manilow's "Jump Shout Boogie," The Marvelettes's "Please Mr. Postman," and The Supremes' "Where Did Our Love Go." 75 remained after this round.

In the last round, they performed a cappella. After their performances, the contestants were divided into four groups in separate rooms. Two groups were eliminated and the number of contestants was cut to 44. After further deliberation by the judges, 24 semifinalists – 12 men and 12 women – were then selected. At the Pasadena Civic Center, each contestant took the elevator from their holding room to the floor above to see the judges, who then revealed their fate.

The rules for this season's semifinal round were changed so as to result in an equal number of male and female finalists. Twelve men and twelve women competed separately on consecutive nights, with two from each group being voted off each week until twelve finalists were left: six men and six women.

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• Carrie Underwood (born March 10, 1983, in Muskogee, Oklahoma; 21 years old at the time of the show) was from Checotah, Oklahoma, and she auditioned in St. Louis with Bonnie Raitt's "I Can't Make You Love Me." She performed Candi Staton's "Young Hearts Run Free" in Hollywood.
• Bo Bice (born November 1, 1975, in Huntsville, Alabama; 29 years old at the time of the show) was from Helena, Alabama, and auditioned in Orlando with The Allman Brothers Band's "Whipping Post." He performed The Box Tops's "The Letter" and The Temptations' "Get Ready" in Hollywood.
• Vonzell Solomon (born March 18, 1984, in Baxley, Georgia; 20 years old at the time of the show) was from Fort Myers, Florida, and auditioned in Orlando with Aretha Franklin's "Chain of Fools." She performed Whitney Houston's "How Will I Know" in Hollywood.
• Anthony Fedorov (born May 4, 1985, in Yalta, Ukraine; 19 years old at the time of the show) was from Trevose, Pennsylvania, and auditioned in Cleveland with Jon Secada's "Angel." He performed Diana Ross' "When You Tell Me That You Love Me" and "Angel" again in Hollywood.
• Scott Savol (born April 30, 1976, in Cleveland, Ohio; 28 years old at the time of the show) was from Shaker Heights, Ohio and auditioned in Cleveland with The Carpenters' "Superstar." He performed the Four Tops's "I Can't Help Myself" in Hollywood.
• Constantine Maroulis (born September 17, 1975, in Brooklyn, New York; 29 years old at the time of the show) was from New York City and auditioned in Washington with Aerosmith's "Cryin'." In Hollywood, he performed The Box Tops's "The Letter" and the Four Tops's "I Can't Help Myself."
• Anwar Robinson (born April 21, 1979, in Newark, New Jersey; 25 years old at the time of the show) was from East Orange, New Jersey, and auditioned in Washington. He performed the Four Tops's "I Can't Help Myself" in Hollywood.
• Nadia Turner (born January 11, 1977, in Miami, Florida; 28 years old at the time of the show) auditioned in San Francisco with Aretha Franklin's "Until You Come Back to Me (That's What I'm Gonna Do)."
• Nikko Smith (born April 28, 1982, in San Diego, California; 22 years old at the time of the show) was from St. Louis, Missouri, and auditioned there with Stevie Wonder's "All I Do." In Hollywood, he performed The Temptations' "Get Ready."
• Jessica Sierra (born November 11, 1985, in Tampa, Florida; 19 years old at the time of the show) auditioned in Orlando with Etta James' "At Last." In Hollywood, she performed Diana Ross' "When You Tell Me That You Love Me."
• Mikalah Gordon (born January 14, 1988, in Las Vegas, Nevada; 17 years old at the time of the show) auditioned in Las Vegas with Ella Fitzgerald's "Lullaby of Broadway" and Lauryn Hill's "Killing Me Softly with His Song." She performed Dusty Springfield's "You Don't Have to Say You Love Me" in Hollywood.
• Lindsey Cardinale (born February 5, 1985, in Hammond, Louisiana; 19 years old at the time of the show) was from Ponchatoula, Louisiana, and auditioned in New Orleans with Karla Bonoff's "Standing Right Next to Me." She performed The Supremes' "Ain't No Mountain High Enough" in Hollywood.

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During the season, Corey Clark, a second season contestant who was disqualified for having an undisclosed arrest record, alleged in an interview on ABC's Primetime Live that he had had an affair with Paula Abdul while on the show.^[10]

During the top 11 competition show, three of the contestants had their voting telephone numbers improperly displayed. The Wednesday night show, which was supposed to have been the elimination night, allowed viewers to vote again as the numbers were fixed, and the elimination took place on Thursday.

Bo Bice was revealed to have been arrested on a felony cocaine possession charge in June 2001 in Madison County, Alabama, and was also arrested two years later for marijuana possession, public intoxication, and possession of drug paraphernalia.^[11] Scott Savol was also revealed to have been convicted of a misdemeanor charge of disorderly conduct for domestic violence.^[12] However, no action was taken against either contestant by the show producers because they had revealed their misdemeanors to them in advance.^[13]

The fourth season of American Idol had an overall average viewership of 26.8 million and was the top show for the 2004–2005 TV season. Its Tuesday episodes averaged 27.32 million (ranked first) while the Wednesday episodes averaged 26.07 million (ranked third).^[14] It also drove Fox, for the first time ever, to become the top network in the 18-to-49 demographic for the season.^[15][16]

Note 1: The Top 11 Wednesday night was a repeat performance night due to a mix-up in the phone numbers for the contestants. The result show was moved to Thursday.

The compilation album for this season was performed by the top twelve finalists.

• American Idols LIVE! Tour 2005