Sometimes the troglodyte approach to fixing keyboards works: just bang
on the bad key for a while, and it may start to work again.  Bang
gently, of course -- don't take "troglodyte" too literally.  :0)
Also, on some types of keyboard, you can get at the contacts just by
pulling off they keycap.  I have no idea if any Model 4's used that
type, though.  Quite likely not.
--
Tim Mann  tim at 
tim-mann.org  http://tim-mann.org/