Basicity of hypoboric acid
This is a Lewis acid, not a Brønsted acid. Like boric acid $\ce{B(OH)3}$, which form an adduct $\ce{B(OH)3(H2O)}$ which then lose a proton from the water molecule of the adduct: $$\ce{B(OH)3(H2O) -&gt; B(OH)4- + H+ }$$ 

Above molecule will do this at each B atom, and so lose 2 $\ce{H+}$ in total, not 4. IE will have:
$$\ce{B2(OH)4 + 2H2O -&gt; B2(OH)6^{2-} + 2H+}$$
as the overall reaction.
Note $\ce{B(OH)3}$ acts as a Lewis acid due to vacant B p orbital, which can accept lone pair of $\ce{H2O}$ and so form an adduct. From this adduct lose proton, and hence acidic.