I've done it. I've bought a Photoshop CS 4 extended Version. But it's a student version.
At first I had tested a trial version and was a little upset. The new functions like rotating the workspace and so on don't work properly. But then I found out, that with an update it worked quite well.
For some curiosity Adobe don't include the updates in their trials. But now it works and I'm happy. I'm looking forward to draw some new pieces with it.