How to read 4 motors' velocities

Hi, all

I am doing an experiment to let the one of 4 main out(motor speed) as an input, and then, output a servo control with respect to that.
For example, The servo can be controlled by the changing speed of the brushless motor.
Setting a mixer seems not possible because only roll, pitch, yaw and throttle can be treated as the inputs. So, I can not read the individual motor speed.
Does anyone have any Idea?

Thanks!
Jin

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