I come from the 3D printing community where it is common to control 12V 2 or 3 pins fans with PWM. 4 Pin fans are quite rare and even old PC mother boards used to use 3 pin. Having taken fans apart, they don't usually have any capacitors in them, just a hall effect chip and some coils. If the frequency is low enough any fan will work with PWM since it is just interrupted DC. I.e. power a fan with 5V and it spins. Disconnect the power and it starts to slow down. It doesn't stop immediately because of the rotor inertia so if turn it on and off at the right rate it will run slow. Fast enough to be smoothed by the inertia, not too fast to upset the electronic commutation.