Hi,
Silverlight 4 is not officially supported even by the current version of TestComplete, 7.52. We are planning to implement special support for applications based on Silverlight 4 in one of future versions of TestComplete.
TestComplete uses its UI Automation engine to test applications based on earlier versions of Silverlight. Several our customers were able to make TestComplete 7.52 work with Silverlight 4 applications. So, I suggest that you update your product and give it a try. You can read the
"Testing Silverlight Applications with TestComplete" help topic for more information.