Yes, I recommend starting from 5.0 - once you have this lot done, you
can shift over to only building the latest sources against these.
You can get mock from here :
http://centos.karan.org/el4/extras/stable/SRPMS/mock-0.6-4.el4.kb.1.src.rpm
- that should just rebuild cleanly on you centos-4 install. Also, stick
with the mock-0.6* tree, anything more recent than that has issues. Once
you have this installed, sanity check /etc/mock/* There are a few
examples there that should put you on track to writing up a new config.
Just make sure you are using the buildsys-build from dev.centos.org.