This issue was spotting by someone in the General forum in response to a question I had:

http://forums.bistudio.com/showthread.php?p=1353004

I tried reassigning my bindings as follows so that I could toggle both crouch and prone just using the Mouse4 button:

Up: Mouse4
Prone: 2 x Mouse4

However, if I either tap or double-tap Mouse 4, it performs "up" - double-tap does not toggle prone.

The following worked correctly.

Up: Mouse4
Prone: Mouse5