I, too, have been waiting for years for DG to reissue this cycle. I think it was available for about a day when it was released, and now it's almost impossible to find. I had contacted ArchivMusic about the possibility of this being reissued on their Archiv CD series, they wrote back saying it could be possible. That was a little while ago now, and still hasn't been reissued.
I haven't heard the 6th, but I know the 4th, 7th and 9th symphonies. While they maybe aren't the versions I always reach for, I certainly enjoy having them, and would welcome the whole set.
Ben