Hi Mark Bourisaw,
The rich text box works for Silverlight 4. There is nothing specific about the format providers and Docx was one of the first format providers that were developed. It is included in the assembly Telerik.Windows.Documents.FormatProviders.OpenXml. Similarly, the HTML format provider is part of the Telerik.Windows.Documents.FormatProviders.Html assembly.
For general information on the assemblies you need to add references to in order to make use of the different features of RadRichTextBox, you can refer to the third post in this forum thread.
the Telerik team