Download source code, open WPF project with any text editor and change TargetFramework from 4.5 to 4.0 as shown bellow.
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
Then you will be able to open and compile that project in 4.0
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
Then you will be able to open and compile that project in 4.0