Some of the new features are things like more advanced conference moderator options, a syslog client, additional SOAP API hooks, and even more easily have the phone system play your exotic music on hold. Plus more.
One of the impressive features of pbxnsip v4.xx is that there is an attendant console that is web based. This means you can run pbxnsip as a hosted server and provide attendant console without worrying about the client! It can work with Windows, Apple, Linux and...what did I forget? ;-)