Everyone knows the 150% STR mod bonus to damage on using a weapon with 2 hands. What if you are using 3 or 4?
By RAW I don't believe you're able to put more than 2 hands on a weapon, though that's simply because the rules don't expect you to have more than 2 hands IMO.
I think they assume the handle would be too small for more then two hands, even if it was long enough you couldnt supply a noticeable difference in force due to long complicated physics that i am no expert in.