The spec for 4pin fan pwm states:
2.1.1 Fan Input Voltagehttp://formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf
Fan operating voltage shall be within the range 12 V ±5% V.
2.1.4 PWM Control Input Signal
PWM Frequency: Target frequency 25 kHz, acceptable operational range 21 kHz to 28 kHz
Maximum voltage for logic low: VIL = 0.8 V
Absolute maximum current sourced: Imax = 5 mA (short circuit current)
Absolute maximum voltage level: VMax = 5.25 V (open circuit voltage)
This signal must be pulled up to a maximum of 5.25V within the fan.