The standard O'Kane method doesn't use a spread, as such. Instead, you determine where to set your crosshairs and then let loose with a torpedo as "juicy" parts of the ship cross the line. But, at 4,000 yards, the accuracy is far more dependent on your scope adjustment. If you're half a degree off at 800 yards ... no big deal. But, at longer ranges, it can mean the difference between a hit and a miss.
Depending on which mods you're using, you can sneak in a lot closer ... as long as you keep your speed at or below 3 knots and only use the periscope for quick peeks. Some skippers will swear you also need to stay deep until the escorts pass by but I've made approaches at periscope depth and gotten inside 1,000 yards of the big boys without being detected. At that range, it's hard to miss.