I used to use 4 when I had bikes with not as much stopping power (my Buell Blast and my F2)... but the reason most MSF courses teach 4 fingers is a safety issue.. if you go down and you're using 2 fingers, there's a chance you could crush the other 2 if the lever gets bent into the handlebar.
The reason *I* use 2 fingers on the Duc is because with those Brembo brakes, any more than 2 fingers and I'll be flying over the handlebars. I've locked up the front once during a panic stop (some idiot backed out in front of me) and that was just with 2 fingers.
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"The Vincent was like a bullet that went straight; the Ducati is like the magic bullet that went sideways and hit JFK and the Governor of Texas at the same time. It was impossible."
- Hunter S. Thompson, Song of the Sausage Creature
"Be who you are and say what you feel
because those who mind don't matter
and those who matter don't mind."
-Dr. Seuss