This is not a really good news. An announcement at OSMC (Open Source Monitoring Conference) as been made by Andreas Ericsson ( Nagios 4 core dev) about the future of Nagios.
Nagios 4 is the latest Core version of the project ! Only the enterprise version will remain (strategic choice of Ethan, the creator).
A Nagios 4 fork will appear called Naemon and developed by Andreas Ericsson. I remind as well that the Shinken is a real good alternative and is able to work with your current Nagios configuration.
So that’s not a really good news for Nagios but will help more highlights on some interesting projects.