The is an electric multiple unit (EMU) train type operated by the Fukuoka City Transportation Bureau on the Hakozaki Line and Airport Line in Fukuoka, Japan, since 2024. Eighteen 6-car sets are to be built.
The 4000 series trains are the first in the world to implement synchronous reluctance motors on a full scale; they are expected to offer 20% energy savings over older, induction motor-driven trains. The trains use bogies with steerable axles to reduce noise on curves.
Internally, the trains use longitudinal seating throughout. With a per-person seat width of , the 4000 series features the widest seats of any commuter train in Japan.
The trains operate on the Airport Line and the Hakozaki Line, as well as the JR Kyushu Chikuhi Line through services.
In June 2021, the Fukuoka City Transportation Bureau announced that it was seeking out manufacturers to build a fleet of 108 new vehicles to replace its fleet of 1000N series trains. It was announced on 7 February 2022 by Kawasaki Heavy Industries that its rolling stock division, Kawasaki Railcar Manufacturing, would produce the new trains at a cost of approximately .
Details of the trains were first announced by the Fukuoka City Transportation Bureau on 30 November 2023.
The first set, 25, was delivered from the Kawasaki Railcar Manufacturing plant in Hyogo from 23 April 2024. By October 2024, two sets had been delivered.
The type entered revenue service on 29 November 2024.
In May 2025, the 4000 series received the Laurel Prize, presented annually by the Japan Railfan Club.