You can use stable ebuilds just remember to sync your portage before you begin.
Remember: All commands are executed as root.
We need to allow the following keyword masked ebuilds.
echo 'dev-util/cmake' >> /etc/portage/package.keywords
echo 'dev-cpp/clucene' >> /etc/portage/package.keywords
Make sure you have set the berkdb USE flag for redland, otherwise nepomuk won't work.
echo 'dev-libs/redland berkdb' >> /etc/portage/package.use
echo 'x11-libs/qt accessibility' >> /etc/portage/package.use
These are the packages you will need to emerge, I included the update option into the emerge command so you will not re-emerge anything that you might already have installed.
emerge -avu 'sys-devel/gcc' \
'dev-util/subversion' \ 'dev-util/pkgconfig' \ 'x11-base/xorg-x11' \ 'virtual/glut' \ 'media-libs/mesa' \ 'media-libs/jpeg' \ 'media-libs/libpng' \ 'media-libs/giflib' \ 'dev-cpp/clucene' \ 'dev-util/cppunit' \ 'media-libs/liblrdf' \ 'dev-libs/libxml2' \ 'dev-libs/libxslt' \ 'x11-misc/shared-mime-info' \ 'dev-libs/boost' \ 'dev-util/cmake' \ 'dev-libs/redland' \ 'sys-apps/dbus' \ 'sys-apps/hal' \ 'x11-libs/qt'
You will also need to emerge either 'kde-base/kdesdk' or kde-base/kdesdk-scripts'.
If you emerged DBUS, CMAKE, QT or HAL you may skip those sections respectively. Good luck!
Also you can install the KDE 4 packages directly via
emerge -a <packagename>
To get the things you need, install layman and then pull in the "kde" overlay (which contains experimental KDE ebuilds):
layman -a kde
After this you need to adjust some USE-flags for KDE 4 and tell portage to use the testing KDE 4 ebuilds instead of the stable KDE 3 ones.
That way portage will do the dependency tracking for you.