PowerShell v4 is now released in the Windows Management Framework version 4.0 for downlevel OS’s, but a number of people have had issues with the install. The WMF package installs successfully with no errors, however, if you check the $PSVersionTable it does not show as v4.0. This is because the .NET Framework 4.5 is a prerequisite. This is present on Server 2012 and R2 of course, but not necessarily on downlevel machines. Unfortunately, a lot of the documentation did not originally reflect this and so customers were left confused at the end of the installation process.
The good news is that the documentation has now been updated. You can find the KB article here.