The fourth edition of the ECMAScript language (ES4) represents a significant evolution of the third edition language (ES3), which Ecma approved as the standard ECMA-262 in 1999. ES4 is compatible with ES3 and adds important facilities for programming in the large (classes, interfaces, namespaces, packages, program units, optional type annotations, and optional static type checking and verification), evolutionary programming and scripting (structural types, duck typing, type definitions, and multimethods), data structure construction (parameterized types, getters and setters, and meta-level methods), control abstraction (proper tail calls, iterators, and generators), and introspection (type meta-objects and stack marks).
ES4 also upgrades ES3 in small ways by fixing bugs, improving support for regular expressions and Unicode, supplying richer libraries, and adding lightweight facilities like xype-discriminating exception handlers, constant bindings, proper block scoping, destructuring binding and assignment, succinct function expressions and definitions, and array comprehensions.
Check the proposed ECMAScript 4th Edition.
Here is also some notes from mook.org which describes main differences.