This product is either discontinued or we cannot get that from suppliers.
Noctua NF-A4x20-5V-PWM 40mm 5V 5000RPM PWM, Fan 40x40x20mm, Rotational Speed (±10%) 5000RPM, Airflow 9.4 m3/h, 8.3 m3/h (with L.N.A.), Acoustical Noise 14.9 dBA, 12.2 dBA (with L.N.A.), Bearing SSO2, 4-Pin Power Connection.
Featuring advanced aerodynamic design measures such as Flow Acceleration Channels and an AAO frame, the NF-A4x20 5V PWM is a PWM enabled 5V variant of Noctua’s highly optimised, premium quality quiet 40x20mm fan.