You can get PowerShell 4.0 for down level operating systems now via the WMF 4.0 download. NOTE: Be sure you have .NET 4.5 installed *before* you install WMF 4.0. For various reasons, the WMF 4.0 installer doesn’t alert you to this pre-requisite which can cause PowerShell to indicate it is still version 3.0 after installation i.e. you didn’t get a good install.
