Have a look in the manual, what i think you are looking for is the 'Trigger Offset' setting.
On the Fly (should be the same on the 4), you need to pop the grip off so you can get at the blue and yellow internal buttons. Press the yellow button to cycle through the menu until you see T1 (or simular) then use the blue button to adjust the 'offset' also known as the debounce.
Have a look in the manual dude, they be rather helpful