The best guide I found for installing Nagios on Ubuntu 16.04 (Xenial) was How To Install Nagios on Ubuntu 16.04, with only a couple of amendments. First of all, it's not using the absolute latest version of Nagios or the Nagios plugins. And secondly, it's missing a command in terms of untarring the downloaded Nagios.
So, as of this writing, the first part of step #4 should be:
wget -c https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.2.1.tar.gz
tar -zxvf nagios-4.2.1.tar.gz
wget -c https://nagios-plugins.org/download/nagios-plugins-2.1.3.tar.gz
tar xzf nagios-plugins-2.1.3.tar.gz
./configure --with-nagios-user=nagios --with-nagios-group=nagios