The proposed final draft (PDF) of the Servlet 4.0 spec has just been made available at GitHub

The major new feature is HTTP/2 support and specifically the push support that comes with it. Java EE already has support for push via WebSockets (including WebSocket support in JSF 2.3), but there are other interesting changes as well, such as for instance the Mapping Discovery API.

The following contains a terse list of changes, taken from section A.1 of the above linked Servlet 4.0 PFD document. All mentioned references to sections are to that document.