Microsoft announced the .NET Framework 4 on 29 September 2008. The Public Beta was released on 20 May 2009. Some focuses of this release are:

* Parallel Extensions to improve support for parallel computing, which target multi-core or distributed systems. To this end, they plan to include technologies like PLINQ (Parallel LINQ),[32] a parallel implementation of the LINQ engine, and Task Parallel Library, which exposes parallel constructs via method calls.
* New Visual Basic .NET and C# language features, such as statement lambdas, implicit line continuations, dynamic dispatch, named parameters, and optional parameters.
* Full support for IronPython, IronRuby, and F#.
* Support for a subset of the .NET Framework and ASP.NET with the "Server Core" variant of Windows Server 2008 R2.
* Support for Code Contracts.
* Inclusion of the Oslo modelling platform, along with the M programming language.
* Inclusion of new types to work with arbitrary-precision arithmetic (System.Numerics.BigInteger) and complex numbers (System.Numerics.Complex).

On 28 July 2009, a second release of the .NET Framework 4 beta was made available with experimental software transactional memory support. Whether this functionality will be available in the final version of the framework has not been confirmed.

On 19 October 2009, Microsoft released Beta 2 of the .NET Framework 4. At the same time, Microsoft announced the expected launch date for .NET Framework 4 as the 22 March 2010. This launch date has subsequently been delayed to April 12, 2010.

On 10 February 2010 a release candidate was published: Version:RC

On 12 April 2010 the final version of .NET Framework 4.0 was launched alongside the final release of Visual Studio 2010.

In conjunction with .NET Framework 4, Microsoft will offer a set of enhancements, codenamed Dublin, for Windows Server 2008 application server capabilities. Dublin will extend IIS to be a "standard host" for applications that use either WCF or WF.