Having .Net 4 doesn't mean you cannot install .Net 3.5. It doesn't mean you will need to downgrade either, as explained in other topics, .Net Framework is a stand-alone software, you can install any version you like without creating conflicts and each app will look for it.
Making a launcher on .NET 4 would mean to drop all the support of .NET 3.5, and that really depends of Rizz availability but atm I don't see any reason to do it.