@bearer Hello, I managed to get the fan speed with this command: M950 F0 C"!out4+out4.tach"
The value reported by DWC is around 4670 RPM which corresponds to the (+/- 10%) 5000 RPM announced in the specs.
So this Noctua seems to be too slow for my needs, I'll change the design of my printer to use a PWM radial fan instead.
Anyway, thanks @bearer & @Phaedrux for your help.
Have a nice day,