I just bought a new fan motor and capacitor. The fan motor is A.O. Smith 1/4 HP. The original had three wires and this on has 4. I have the 2 brown ones hooked up right to the capacitor. I am confused on where to hook up the black and yellow. I think the black goes to where the old black one went but not sure. I do not know where the yellow goes. I need to know if I need to install the old red wire that went from my contactor to the capacitor back to the new capacitor. Thanks

In a 4 wire system the capacitor should have two wires connected to it.

One of the hots will go to the contactor and it's probably black. The other is common and should go to the wire feeding the side of the old capacitor that had two wires on it.

The diagram should be on the motor.

To the HVAC moderator: This question seems too common and might be replaced by a better answer with pictures and a sticky.