Probably the most interesting change is the addition of CQLinq – a LINQ language for querying the code base that supersedes the old “CQL” query language (that was more SQL-like).
It also includes support for Visual Studio 11 and a new API to make creating your own static analyzer(s). There’s a full list of new features in the Release Notes.
Upgrades from v2/3 are half the cost of a v4 license.
If you haven’t upgraded from v3 to v4, you can still download the latest v3 from the downloads page (it will say that your v3 license key isn’t valid for v4 but give you a link to the v3 download)