On the page labelled:

Getting Started/Build/Stable Version
Building KDE3 From Source

Under the heading:

Installation

It says: "...append $QTDIR/bin and $KDEDIR/bin to your $PATH..."However, if you are running on a machine that already has an old version of QT installed, this will cause it to find that old version, instead of the new one you just made.
Instead, you need to PREPEND the new paths to $PATH. This makes it work.

--Phil