After installing .NET Framework 4, IIS 6 is not directly enable the .NET 4. Run this command to check :
cd \windows\system32\iisext.vbs /ListFile
I got a problem when enable .NET 2 and .NET 4 simultaneously. So, I disable the .NET 2 and enable the .NET 4, by run this command :
C:\WINDOWS\system32>iisext.vbs /disfile C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll
C:\WINDOWS\system32>iisext.vbs /EnFile C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll
Restart IIS and the .NET 4 should be work.