Reply from PMDG: "As we finalize each of the P3D v4 product updates, we are reissuing them in a new installer that is capable of handling installations to either P3D v3 or P3D v4, similar to the approach we took with FSX/FSX-SE."   So we should have the new installers online  the moment they are ready. For boxed users we'll have an upgrade path too, of course.