Thank you for writing.
We do provide two builds of our assemblies - one for .NET2.0 and one for .NET4.0. The first is in the Bin\ folder while the latter is in the Bin40\ folder. Most of our controls are built using 2.0, except the RadRichTextBox and RadSpellChecker components which require 4.0, which are only available with the 4.0 version of the assemblies.
Feel free to use either version of the suite your projects.
I hope that the provided information addresses your questions.
the Telerik team