I have heard the Mravinsky/Leningrad 4th, and I would call it good, not great. For the 4th, I would recommend Rozhdestvensky/LSO, Jansons/Oslo, and Abbado/VPO. I'm still looking for an outstanding 5th. For 6, I do love the Mravinsky/Leningrad, but I also love Reiner with Chicago.