A few months ago, Andreas Ericsson, the main developer of Nagios 4, has been kicked from the Nagios Developer Team for personal reasons. So he decided to continue development in a new fork called Naemon. The result so far is quite impressive.
Right after closing the Nagios users and developer mailing lists, Andreas got kicked so it seems like someone did not want give people a place to discuss about such behaviour. Although the official Monitoring Plugins Team has already setup replacement nagios mailing lists.. At that point nobody knew that the Plugins Team will be kicked later too, but thats a different story.
Since then, a lot of things have already been done to make Naemon the better alternative to Nagios 4.
So far, the Naemon Developer Team consists of 4 people already, and there are likely more to come. Especially since there is a break in between the community and Nagios Enterprise.
There are several resources if you are interested in Naemon:
Do not use in production yet, the project just has started. But there will be a stable release soon.