Following MIX 2010 annoucement, Microsoft confirmed that “.NET 4 will be available in Windows Azure within 90 days of .NET 4 RTM. As part of our preparation for that, latest operating system build available in Windows Azure contains .NET 4 RC. Although you cann’t use this build to run .NET 4 apps, please let us know if having .NET 4 RC in the build has any effect on your existing apps. One known effect you may see if you’re consuming generic ASP.NET performance counters is that they’ll report data only on .NET 4 applications. You can instead use versioned performance counters as documented in KB article 2022138. As always, you can choose which build of the OS your app will run on in Windows Azure,” said Microsoft. See MSDN documentation for details.