On this old question How to install Octave 4.0.0 in Ubuntu 14.04 they say to use this:
sudo add-apt-repository ppa:octave/stable
sudo apt-get update
sudo apt-get install octave
But when I run this command on Ubuntu 18.04:
$ sudo add-apt-repository ppa:octave/stable
[sudo] password for linux:
HTTP Error 404: Not Found
Cannot add PPA: ''This PPA does not support bionic''.