Unfortunately, XIDE hides the .NET Framework selection from the user and uses the newest one available on the developers machine, but fortunately it is possible to change - maybe to build an executable that runs on Windows XP where only .NET 4.0 is available.
Simply follow these steps:
/nostdlib(it akes the compiler not automatically add a reference to mscorlib.dll)
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework. Just pick the folder of the framework version that you want to use and add references to those files, including mscorlib.dll.
Maybe somewhere in the future there will be a plugin for this if enough people asks for it.