Hmm, firstly, make sure you are trying to download the demo from this page: http://fxhome.com/products/demo
Depending on whether you want the standard or Pro version, and whether you have a Mac or a PC, select the appropriate button from that page and click it. The installer for the demo of your choice will then download.
Now, if you have the PhotoKey 2 demo still installed on your machine, delete it. After its deleted, run the installer for the version 4 demo. Hopefully that will get you up and running.