Hmmm... I think it would not be a good idea to add a RHEL4 repository, since a lot of things could have been changed between the two releases and this may result in some unpredictable behavior. Anyway, can you tell me the name of the package who provides the existing libX11.so? You can try
Code:
rpm -qf /usr/X11R6/lib/libX11.so
or whichever the location of the shared library is.