There are two ways to change the default Kde4 window manager. One involves creating a script that changes a global variable and the other involves using Kde system settings. While both methods work I always find solutions involving working in a terminal to be simpler and faster. Still I will be detailing both methods to the best of my knowledge.
Update: Recently it has become quite popular to try out the new opengl es mode of kwin. Which was originally aimed to bring proper compositing to mobile platforms and is paving the way for compositing on wayland. Currently it seems that this new code will eventually replace the old opengl code. The reasons to why this is so interesting is that it supposedly brings a few perfomance improvements. I've yet to confirm this, but it does run fine on my netbook using intel drivers. But on my desktops using Nvidia graphics it fails miserably, this could be beceause of the code or because of the Nvidia drivers, who knows. Anyway it is worth trying out, but keep in mind that your mileage may very greatly depending on what kind of hardware and drivers you are using.
Have a look at Martin Gräßlin's blog for further details.
http://blog.martin-graesslin.com/blog/2011/07/running-kwin-with-opengl-es-2-0/