4 will be quite different, and the ease of installing and running an out of the box example in combination with documentation is not quite ready. So I think you have to be a little patient for a short while (2 months until 4.0 will be GA)
You can download the beta zip from http://sourceforge.net/project/showfiles.php?group_id=70542. Unzip it, and run the installer. You should get a doc folder containing the userguide and the developers guide.
As Ronald states, when v4 goes GA, the idea is to have a zip you can download, unzip and start playing with (as was the case with jBPM3).