I'm replacing an a/c fan motor of three wires with one that has four wires... The new one has two brown wires that I assume go on the new capacitor of 7.5uf... with a yellow and black wire left. Where do these wires go? On the old 3.5 capacitor, or on the other panel of the unit..

Yes both browns to the new capacitor. The other two go to either side of the contactor that starts the unit. Usually this is called the load side since the line side of the contactor is fed 240 volts at all times. When the contactor makes the voltage goes to the motor and it starts along with the compressor.

To hvac 1000... thanks man.

My old motor had only one line, it was a blue wire on that contactor point... Do I place the yellow wire there, and the black wire on the next terminal? And... since my compressor is on the old dual capacitor, do I just leave it alone?