Which release should I install?
Since the original release of .NET 4.0.0 in April 2010 there have been multiple updates. By Dec 2011 the current release was 4.0.3.xxxx and this was the only release available on the download center.
In Visual Studio if you target .NET 4 your code will run on .NET 4.0.0 or higher.
If you target .NET 4 Platform update 1 your code will run on .NET 4.0.1 or higher.
If you install .NET 4 in December 2011 or later you will get .NET 4.0.3 so either release will work.
If you install with NuGet you will get both releases.