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:

Contents 1 openSUSE 2 Arch Linux 3 Kubuntu 4 Gentoo 5 Fedora

[edit] openSUSE

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/.

Instructions: http://opensuse.org/KDE4

[edit] Arch Linux

Arch has daily updates of KDE's trunk (including playground and extragear).

Instructions: http://bbs.archlinux.org/viewtopic.php?id=44507

[edit] Kubuntu

Kubuntu packages all the development snapshot releases of KDE4, and these are released in different repositories. The latest one is below.

Instructions: http://kubuntu.org/announcements/kde-4.0.1.php

[edit] Gentoo

Official Gentoo KDE Team testing overlay provides packages for KDE 4.2 live branch, unstable snapshots, and KDE's trunk as well.

Instructions:
1) Visit http://forums.gentoo.org/viewtopic-t-708282.html for more information about overlay support.
2) # layman -a kde-testing
3) Visit Documentation/ directory in overlay for more information about setting up your system.

[edit] Fedora

Fedora Project has released packages for KDE 4.0.0 through KDE repository (http://apt.kde-redhat.org/apt/kde-redhat/fedora/kde.repo)

Instructions:
1) Download the KDE repository from the link above.
2) Enable KDE-unstable repository.
3) Do a "yum update".