There are 3 hydration forms of sodium metaborate' as evidenced by 3 CAS numbers
Anhydrous (NaBO2) CAS 7775-19-1
A dihydrate (NaBO2.2H2O or as Na2B2O4.4H2O ) CAS 16811-11-6
A tetrahydrate (NaBO2.4H2O or as Na2B2O4 .8H2O) CAS 10555-76-7
Things are made rather confusing by representing the anion in two different forms BO2- or as B2O4-2. Once the anion is represented the same way it is easy to see the different forms.
The tetrahydrate converts to the dihydrate above 53.6C (128F).