Maybe you're wrong?
What are the weak points of the album in your opinion?
Vol 4, for the most part, sticks within the terrain settled by
Paranoid and
Master of Reality. At its best, i.e. "Wheels of Confusion," the opening riff of "Cornucopia," and "Under the Sun," it results in some rather epic doom. However, it doesn't offer much new. There are a few hints of the progressive elements that characterize the next four releases: the latin-esque passage on "Supernaut" and the glitzy synth at the end of "Snowblind," but at this point they're only hints.
I wouldn't say any of the metal tracks are bad; they're all at least solid. However, too often, Sabbath seem to be going through the motions with songs that are adequate, but lack both the distinctness or memorability of the tracks from the prior albums.
As for the non-metal tracks, they're not Sabbath's best. "Changes" is OK, if you're in the mood for an sad sod ballad, but certainly doesn't even come close to the magic of "Caravan" or "Solitude." Also, "FX" is an absolutely terrible piece of experimental ambient. I know a lot of Sabbath albums have folk or ambient interludes, but none is so dull as "FX." Putting these two tracks back-to-back was a mistake, as it makes a long lull within Side A.
All in all, it's still a good album, and I would give it a B if grading it, B- at my most cynical. However, its easily the least engaging and least important of the first six albums.