Sodium metaborate has two hydrates. The confusion arises from how these hydrates are represented in formulas. Often the formulas are doubled making the tetrahydrate look like the octahydrate.
tetrahydrate NaBO2.2H2O also as NaB2O4.4H2O
octahydrate NaBO2.4H2O also as NaB2O4.8H2O
To distinguish between the two it is imporatant to not only look at the number of water molecules but also the number of boron atoms.
20 Mule team Borax is quite pure and can be used for photography. The term photo grade is misleading as it can be quite impure chemically. The only thing that photo grade gets you is a guarantee that the chemical contains no compounds the are injurious to the photographic process. It can still contain many impurities.