Is there any document or manuals how to upgrade from version 2.8.3 to version 4?
Upgrade is very easy but before you upgrade make a backup of your whole website root and from your database.
Make backup of your whole website and database.
Move the content of the new version of CB to your website directory/root replace the old files with the new files
Call YourDomain.TLD/cb_install you will than asked for upgrade.
But before you upgrade your live site check the new requirments
You need PHP 7.x or 5.6 for the new cb version. create an subdomain as named beta.yourdomain.TLD upload there the new cb version setup than for the subdomain PHP 7x or 5.6 and check if it is working if not ask the discourse community or cb staff.