!!! Don’t forget make Snapshot or Backup from Your system !!!

I have used Ubuntu 14.04 and Nagios Core 4.0.8, and this steps worked, I tried!

# sudo htpasswd /usr/local/nagios/etc/htpasswd.users newusername
# /etc/init.d/apache2 reload

edit nagios cgi.cfg file, which located maybe here

# sudo nano /usr/local/nagios/etc/cgi.cfg

you will found this part in the cgi.cfg, and add next to it with comma

authorized_for_all_services=nagiosadmin,newusername
authorized_for_all_hosts=nagiosadmin,newusername

You have to restart nagios

# sudo service nagios reload

after reload you can able to login into nagios web page by newuser, and you have read only rights.