"One 4 U" (Japanese: ワン・フォー・ユー Wan Fō Yū; Korean: 원포유 Wonpoyu) is a song by South Korean boy band A-Jax. It was released on June 1, 2012 as the group's debut single. The song "Never Let Go", used as B-side for the Korean single, was released on May 15, 2012, two weeks before their official debut, as a prologue single. The song was re-recorded in Japanese and will also serve as their debut single in Japan. It was released in August 8, 2012.

On May 7, a mysterious video featuring Kara's member Goo Hara was revealed on DSP Media's YouTube account with a male voice on the background. The video created a confusion because it was not revealed if the video was a teaser for an upcoming Korean comeback of Kara, or a teaser for an upcoming solo event of Hara, or something related to A-Jax. On May 10, another video of Hara was released, revealing that the teaser was related to A-Jax. In the teaser, a song of the group (at this time the name was not revealed) was playing at the background. On May 15, DSP Media released the song and music video of the song played on the teaser, called "Never Let Go". On the music video only appears Hara. Later it was stated that the song was only a prologue song. On May 21, it was revealed that A-Jax was set to be the opening act in Kara's last two shows of their first tour, Karasia, in Saitama, Japan. The members stated "It's an honor to be able to stand on such a big stage before our debut. It'll be a learning experience for us. As it is a great opportunity, we're going to work to show as much of our charm as possible." On May 1, it was revealed that the group will be debuting in South Korea and Japan simultaneously in 2012. It was also revealed by DSP that "A-Jax will be the first Korean idol group to debut within Korea and Japan simultaneously, and will follow in the footsteps of the group SS501. A-Jax has signed under the equivalent conditions that other Korean male idol groups, who have already established their popularity, have been given, making it an unprecedented event. Thus, they are receiving much attention from Japanese music industry insiders." For their Japanese releases, they signed with the label Universal Sigma, where their labelmates Kara and Rainbow are currently signed.

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