There is a note available on the Silverlight 4
site[
^].
"This is a developer beta release only! This release of Silverlight 4 Beta is a developer release. What this means is that there is no “go-live” licensing available and the end-user runtime of Silverlight 4 is not available. The tools below are intended for software developers only."
So you wont be able to release your product to the client.