I recommend this tool:
http://kdesvn-build.kde.org/
It is the only one I could convince to work.
Though I am not sure I understand I you mean saying that you downloaded KDE 4.0 RC 1. Is that an archive with sources, or is it some binary installer, debian archive or rpm?
If that is an archive with sources, read the instructions that come with the script I recommended and you will be able to build them.