Painter Essentials 2 and Painter Essentials 4 are designed to run on Windows XP. You can try to install on your PC by using the Compatibility Center available in Windows 7. On the following web page you can find a tutorial about how to use this feature.
However, you cannot upgrade to Painter Essentials 4 from Painter Essentials 2 for free. You can visit the following official web page where you can find more informations about the software as well as a list of prices and a download link.