Actually Silverlight runtime doesn't use GAC during assembly probing, so you can safely remove mentioned assemblies from there.
Are experiencing the problem with any Silverlight project, or only such containing RadRichTextBox control? I will send you a sample project to verify at your side in the support ticket you opened.
You can also check if Silverlight 4 and Silverlight 4 SDK are installed on your computer, as RadRichTextBox references System.ComponentModel.Composition, which is part of the SDK.
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