How do you add the 4.0 reference assemblies to your build server (so the compiler finds them)?

They are already present on the machine. Just change the Target Framework setting of the project from 4.5 to 4.0

The problem discussed in that linked question is a pretty common one for programmers that try to setup a build server without paying for the VS license. Which goes pretty far, but is missing an otherwise freely available component, the multi-targeting packs for the .NET Framework versions.

Which is penny-wise but pound-foolish. They then make a fatal mistake, they add reference assemblies from c:\windows\microsoft.net instead. Like it was done in framework versions prior to .NET 4.0. This causes horrible to diagnose runtime exceptions when the built product is ran on a machine that only has 4.0 installed. The asker of the question was pretty lucky, he got a relatively easy to diagnose one. That however can be a lot worse, getting pretty bizarre TypeLoadExceptions for common framework types.

You don't have this problem, you installed VS so you already got the multi-targeting packs. The 4.0 version is available in C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0. Both MSBuild and VS know how to find it there without your help. Just change the project setting to tell them that you want to target 4.0