Well that was unexpected!

I proceeded to install PowerShell 4 and all seemed fine... except where was PS4? This still looks like PS2.

Eventually I stumbled across a TechNet article that exactly described my problem. It seems that PS4 requires the .NET Framework 4.5 as a prerequisite. If you don't have .NET 4.5, you will get no errors or warning messages. The PS4 installer will not add .NET 4.5 for you, oh no. Instead the install will appear to run successfully to completion, with only the small problem that you won't, you know, ever get PS4.

See section "Installation succeeds even if .NET 4.5 is not installed" in the link below:

http://social.technet.microsoft.com/...shell-4-0.aspx

The solution is that you have to manually install .NET 4.5 yourself, then you can run the PS4 installer, and all will be good.

You know, it's like not having an install program at all. Who needs this logic and prerequisite checking stuff anyway?'