ESRI has announced that ArcGIS 10.4 prerelease software for Desktop, Server, and Engine is now available for you to download, install, and test. They do stress it is for testing purposes only, so I would not use it in production.
Some highlights are vector tile layers, Portal for ArcGIS Map Viewer includes analysis tools, and improved enterprise readiness with ArcGIS Server and Portal. Also, ArcGIS Pro 1.2 is currently in beta. For more info about the release, see their blog post here.
UPDATE 2/22/2016: The ArcGIS 10.4 release is now available.