Fedora Core 4 dos not provide kdiff3 package, so you need compile it by yourself:

yum install qt-devel
yum install kdelibs-devel
tar xvzf kdiff3-0.9.88.tar.gz
cd kdiff3
./configure --with-qt-dir=/usr/lib/qt-3.3
make
make install

Then you can found the kdiff3 program at /usr/local/kde/bin/kdiff3.