In theory, the acid anhydride reacts with water to get the corresponding acid.
Note: Not all acid anhydrides will combine with water to form acid. For example, the acid anhydride of acetic acid has "stripped" the water between the molecules, and the "stripped" water cannot be restored by returning the water directly.
SiO2 is the same reason. H4SiO4 is orthosilicic acid [Si(OH)4], not silicic acid. Silicic acid is H2SiO3, so the water removed is SiO2.
The acid anhydride of H4SiO4 is also SiO2, because H4SiO4 is equivalent to H2SiO3·H2O. The combined water is "externally hooked" and can be taken off directly without care. Just like alum KAl(SO4)2·12H2O, the bound water can be omitted directly. Although it is in the chemical formula, it is just a "external hanger", equivalent to wearing a piece of clothing.