Q:

Microsoft Windows SDK for Windows 7 and .NET Framework 4 (ISO) is backward compatible? 

On windows 7 if i install Microsoft Windows SDK for Windows 7 and .NET Framework 4 (ISO)  is that enough to work with vs 2008 or should i install also 3.5 sdk ? 

A:

Quote from the Windows SDK for Windows 7 and .NET Framework 4 release notes: 

.NET Framework:  Create applications    that target the .NET
  Framework    versions 2.0 (SP2) , 3.0
  (SP2) , 3.5    (SP1), 4. 
Visual Studio:  Use the resources in    this SDK with Visual Studio
  versions    2005, 2008, and 2010,
  including    Express editions (not all
  features    work with all versions of
  Visual    Studio). 

Thus yes, includes backwards compatibility into the lines described in the release notes.