Hi Chris Hayward,
Thank you for contacting us.
You can use Sitefinity Migration Tool to upgrade your project to Sitefinity 4.0. Make sure to backup first. Please review the upgrade documentation here.
Sitefinity 4.0 works with license files which have the extension .lic.
This license file is needed by Sitefinity because it shows that particular domain is associated with the website.
Please contact our sales team
to arrange the details around the migration of your 3.x licenses to 4.0
the Telerik team