  We develop new techniques to compute the weighted $L^2$-cohomology of
quasi-fibered boundary metrics (QFB-metrics). Combined with the decay of
$L^2$-harmonic forms obtained in a companion paper, this allows us to compute
the reduced $L^2$-cohomology for various classes of QFB-metrics. Our results
applies in particular to the Nakajima metric on the Hilbert scheme of $4$
points on $\mathbb{C}^2$, for which we can show that the Vafa-Witten conjecture
holds. Using the compactification of the monopole moduli space announced by
Fritzsch, the first author and Singer, we can also give a proof of the Sen
conjecture for the monopole moduli space of magnetic charge $3$.
