!!! 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
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.