Our GeoBase library still targets SilverLight 3, but you can use it for SilverLight 4 applications. We are not sure what you want to achieve. You may try it and see whether it meets your requirements. The trial license should work for SilverStream.
For any technical problem, please write to gbsupport@telogis.com.
Thanks Sharon for your nice feedback, i implemented silverstream demo samples with the silverlight-4, actually i asked it to sure that it will behave as same in the silverlight-3.