While going over the Ubuntu installation guide, I noticed that the install instructions for Graylog are missing in the Ubuntu installation guide
The hint header gives the command installing /just/ the graylog-server package but, there is definitely a codeblock section missing from above the hint.
The link to the .deb is missing along with installation commands.
I believe the install packages for Ubuntu and Debian are the same so, a copy-paste from the Debian installation guide should suffice.
Update, the debian installation instructions seem to be incorrect.
The repository deb file supplied in the installation guide returns an HTTP 404.
Looking at https://packages.graylog2.org/packages - The latest repository DEB file is version 4.3.