At least the
Encyclopedia of the Alkaline Earth Compounds
By Richard C. Ropp
states too that the tetrahydrate is more soluble in water than the anhydrous salt.
Maybe it doesn't imply that, if starting with the tetrahydrate form, the same final water amount contains more beryllium and sulphate.
It can be a matter of definition of the solubility:
- The four water molecules add mass to the amount of the dissolved solid;
- and the four water molecules increase the amount of water available to dissolve beryllium and sulphate;
so in the same initial amount of liquid water, a bigger mass of hydrate dissolves.