Silverlight v4 Beta JPN SDK is released at http://www.microsoft.com/downloads/details.aspx?displaylang=ja&FamilyID=a53019b2-4370-4e8a-a940-518f538c759f

Silverlight v4 Beta JPN SLtools is released at  http://www.microsoft.com/downloads/details.aspx?displaylang=ja&FamilyID=68a158af-9bb8-4855-a724-d09d196eaaf3

If you have Dev10 Beta2 JPN installed, you can install JPN SLTools which will install developer runtime, JPN SDK and tools as part of the setup and you can create JPN Silverlight 4 apps.