[2] In 2022 Mitsui made his single-seater racing debut by competing in the F4 Japanese Championship.

[3] He joined the Honda Formula Dream Project team alongside Syun Koide, and Kazuma Nishimura.

He won the opening race of the season and he collected 2 more wins along with 8 more podiums to finish runner-up to teammate Koide.

[1] For 2023, Mitsui stayed in F4 to fight for the title alongside Yuto Nomura, and rookie Tosei Moriyama.

[4] Mitsui ended up finishing 3rd place in the standings to Runners Up Jin Nakamura, and the eventual champion Rikuto Kobayashi.