Many distributions provide KDE SVN (KDE 4) and Qt packages, so that if you're simply porting your application to KDE4, then you don't have to worry about compiling kdelibs and kdebase frequently in order to keep up-to-date. Here's the information for a few different distributions:
openSUSE's KDE team has very regular (every few days) KDE SVN packages built. Currently packages are built for all things in KDE's branch/.
Arch has weekly updates of kdelibs, kdebase, and kdepimlibs.
Kubuntu packages all the development snapshot releases of KDE4, and these are released in different repositories. The latest one is below.
The kde portage overlay has kde-base/kde*-9999.4 SVN monolithic ebuilds.