I'm replacing my attic fan motor and the old motor only had three wires (red,blk,white ).the new motor has four wires( blk-hi,blu-med,red-low) so can anybody help me to wire this motor.

Where are you located? The existing wiring you describe would is for a 240vac circuit (three wire)

Please give us the manufacturer and model number of the fan you are trying to install.

Old Fan may have been 2 speed? The New one a 3 speed, You could get a 3 speed rotary switch with an Off position. If existing was 2 speed, Black for low, and red for high or Vice Versa. Maybe?
You can just use a single pole switch and use the speed you desire(low, medium, or high).
Seeing how the existing is wired will help a lot, May even have adjustable temp control, When attic exceeds a temperature, it will turn on.

Good Luck

Dayton direct drive blower motor 1/2hp #4M101BG also do I need to add a capacitor and can I connect the control box from the old motor to the new motor and wire it to the low speed

It is probably a 2 speed fan motor. Look at the switch. Last one I replaced had a double throw switch. 1 hot, 2 speed. Motor should show wiring diagram.

You could use s single pole double throw, with a center "off" as mentioned
Common terminal goes to hot, One pole could be 1 speed, the other pole a second speed, May need a rotary for all three speeds