Walter's Mahler 4 is, indeed, available in the recent 77-CD edition; it's also to be found on the earlier Sony 'Bruno Walter conducts', a 7 and 3/4 hour collection, now download only on 88691920102 for around £50. I'm not a great fan of large sets, but, in penance for having overlooked both of these, I'm planning an article on the contents and what remains available on single CDs or smaller sets.