The Divi 4.0 update is a completely free update as long as you have an active account with us.
You can update from within your WordPress Dashboard. Please see: How To Update The Divi Theme
You can find your Username here, if you're not sure.
If you still don't see the update in your WordPress Dashboard, please try checking the Updates page (Dashboard > Updates) instead of the Themes page to update (Appearance > Themes)
Note: Sometimes it can take up to 10 minutes after refreshing the Updates page to see the available update.