why does control-4 generate SIGQUIT in X?

[Eric Cooper <ecc@cmu.edu>]

I recently noticed that when I hold the control key and press '4', it
generates ^\, i.e. SIGQUIT.  This happens in both gnome-terminal and
xterm, and on two different kinds of physical keyboards.  It doesn't
happen outside of X, on just a linux VT.

It's pretty annoying in mutt, because I often need to type shift-4 for
escape and typing control instead of shift kills the session.

What kind of X keyboard mapping determines this behavior, and how can
I change it?

(Please CC me since I'm not subscribed to this list.)

-- 
Eric Cooper             e c c @ c m u . e d u