Does anyone know if there is (or will be) a SSCLI release for the v4.0 runtime?
Not yet, and it doesn't look like it will be there soon.
I get information from the CLR team that they are head down implementations of something, and do not currently plan to release SSCLI based on .NET 4. They recommend using SSCLI based on .NET 2 because it is still a good reference for ECMA CLI.