If it was me, I would uninstall version 4 first.
Download 'aswclear' from the avast website, uninstall version 4, run 'aswclear' and then install version 5.0.396.
Then you should be able to register on-line through the program itself if it's the free version. If you buy the pro version you will get an email with a license file.
HTH, Gizbar.