Can i upgrade the .Net Framework 3.5 to 4 when i have a running Sharepoint(Windows SharePoint Services 3).Whats the impact?

Hi All,
     We are thingking of upgrading the  .Net Framework  3.5 to 4 while an existing Windows SharePoint Services v3 is running.
Is this safe or it will break the installation.
Would there be any impact on the sharepoint instance?

Cheers