If there isn't a rpm for it on your Redhat CD, poke around the Redhat FTP site and its mirrors to get it. Do bear in mind that libncurses-3 and -4 coexist so there is no need to replace one with the other, i.e. a plain rpm -i will do.
Or you might try the following:
cd /usr/lib ln -s libncurses.so.4 libncurses.so.3 ldconfig rpm -i --nodeps kdebase... |
Since libncurses-3 and -4 are 100% compatible both at binary and at source level, installing two versions is just a waste of space.