Task: Question Answering
Input (English): Passage: Throughout the 360° Tour, the band worked on multiple album projects, including: a traditional rock album produced by Danger Mouse; a dance record produced by RedOne and will.i.am; and Songs of Ascent. However, the latter was not completed to their satisfaction, and by December 2011, Clayton admitted it would not come to fruition. The sessions with Danger Mouse instead formed the foundation of U2's next album, and they worked with him until May 2013 before enlisting the help of producers Paul Epworth, Ryan Tedder, Declan Gaffney, and Flood. The band suspended work on the album late in 2013 to contribute a new song, "Ordinary Love", to the film Mandela: Long Walk to Freedom. The track, written in honour of Nelson Mandela, won the 2014 Golden Globe Award for Best Original Song. In November 2013, U2's long-time manager Paul McGuinness stepped down from his post as part of a deal with Live Nation to acquire his management firm, Principle Management. McGuinness, who had managed the group for over 30 years, was succeeded by Guy Oseary. In February 2014, another new U2 song, the single "Invisible", debuted in a Super Bowl television advertisement and was made available in the iTunes Store at no cost to launch a partnership with Product Red and Bank of America to fight AIDS. Bono called the track a "sneak preview" of their pending record.On 9 September 2014, U2 announced their thirteenth studio album, Songs of Innocence, at an Apple product launch event, and released it digitally the same day to all iTunes Store customers at no cost. The release made the album available to over 500 million iTunes customers in what Apple CEO Tim Cook called "the largest album release of all time." Apple reportedly paid Universal Music Group and U2 a lump sum for a five-week exclusivity period in which to distribute the album and spent US$100 million on a promotional campaign. Songs of Innocence recalls the group members' youth in Ireland, touching on childhood experiences, loves and losses, while paying tribute to their musical inspirations. Bono described it as "the most personal album we've written". The record received mixed reviews and drew criticism for its digital release strategy; it was automatically added to users' iTunes accounts, which for many, triggered an unprompted download to their electronic devices. Chris Richards of The Washington Post called the release "rock-and-roll as dystopian junk mail". The group's press tour for the album was interrupted after Bono was seriously injured in a bicycle accident in Central Park on 16 November 2014. He suffered fractures of his shoulder blade, humerus, orbit, and pinky finger, leading to uncertainty that he would ever be able to play guitar again. 
Question: Who enlisted the help of four producers?
Output (English): U2'.