I love Volume 4. It's got the heaviness of Master of Reality in some areas, but it gets more progressive. It's the perfect link between Master of Reality and Sabbath Bloody Sabbath. In Tony's book he called SBS something like the "pinnacle" of Black Sabbath. And I completely agree. It's heavy, sorta progressive, but you can still smell the bluesy jam influence. It also sounds huge, even all this time later.
The only reason I don't think 13 will be able to compete with Volume 4 for me is variety. My impression of 13 from "God is Dead," the bit of "End of the Beginning" on CSI, and from band interviews is that they're really channeling the Black Sabbath-Paranoid-Master of Reality vibe. Which is fine. Those albums are tremendous, and I mean, eventually the variety and progressive elements sort of overwhelmed things on Technical Ecstasy and Never Say Die.
Anyways, I never expected 13 to beat the first six Sabbath albums. But if it can sit along side of them then it will be a win, and if it can easily sit next to them... then it's a big win.
Honestly, it's just kind of awesome that in 2013 there's a bona fide Black Sabbath album being released. (At least Forbidden won't go down as the last "official" Black Sabbath album...)
7/9 - Alice Cooper
8/6 - Black Sabbath