Hey Eagler,
First thing you got to do is get ahead of the fleet (this works for PT's as well as aircraft) and make sure of it's current course. A beam or trailing shot is a waste of time and fish.
Once ahead of the ships you want to position yourself where you are about 45 degrees off their bows.
As there is no sight on the PT use the side of the boat as a reference mark to position yourself. Try to keep your 45 degree angle on the run in. You may have to do some manuvering if they have manned gunners.
Close till you see their auto A/A fire start to shoot at you. The second it does fire your first fish and turn away in the direction of travel OPPOSITE of the ships. As you turn, fire the other 3 fish in a "Fan."
After you fire jump to the stern 40mm gun as you can usually get a few licks in with it before you are out of range. Some times you can knock out the flack gun positions on a ship.
BUT make sure you continue your turn first or you will be running into the destroyer screen.
Hope this helps guys !