Measure RPM from PWM input (4 wire fan)

My apologies if this question has been addressed previously, I wasn't able to find any answers though.

I'm looking to use a 4-wire fan, using a PWM output to control speed (which I assume will be straight forward), but also looking to "read" a PWM input for the purpose of measuring the fan's RPM.

Is there a way to achieve this?

thanks,
Dave