Solaris (and OpenSolaris) are Free Software operating systems released under the CDDL by Sun Microsystems. They are vaguely BSD-like. KDE4 runs on this operating system.
The KDE Project on the OpenSolaris site is intended to be the definitive source of information, but this page on TechBase is intended to collect information, porting and compilation guides, etc. Since TechBase is a wiki, this is much easier than going through the OpenSolaris editing process.
KBE Compilation and Installation
KBE is the "KDE Build Environment" which gives you the tools needed to build the dependencies of KDE and then KDE itself. You can get a tarball of KBE and unpack that. SVN is included in there, which we need later. There is also make and cmake and other bits and pieces; vim is also included to soothe the nerves of vi-using KDE people like me.
/usr/sfw/bin/gtar xvzf KBE.tar.gz
These four commands will fetch and unpack and then start the KBE install script. This script will install dependencies that KBE has which are on the Solaris install media (SUNWi2c and twenty others, I think). Then it will start building the packages for KBE itself, starting with pkgtool. It should go off without a hitch (otherwise post output to the mailing list). If it craps out somehow, you will have to start over from scratch: pkgrm -Y KBE ; bash kbe-install.
Once KBE is done installing, you can check with pkginfo -c KBE to see what it has installed. You will have a /opt/kdebld filled with "stuff". Right. Now source the environment from KBE, which you will use in later steps:
(Use env.csh if that's your poison). If you don't have an env.sh at the end, something is definitely wrong. I end up with 210 or so files in /opt/kdebld/bin, so keep that in mind as a metric. There are 23 packages in the KBE group.
Avoiding KBE dependencies
You can give kbe-install a --nodeps to avoid the SUNW packages that it wants to install. You *do* need to have Sun Studio 12 9/07 or later installed; any later patches are welcome as well. I do not know what avoiding the SUNW dependencies will do, actually: they don't really seem to be essential, but I would suggest skipping this step *only* if you don't have the install media handy.
If kbe-install does not work, please pop into #kde-solaris on Freenode (irc.kde.org works) and ask questions.