  We investigate instability of 4-dimensional Reissner-Nordstr\"om-anti-de
Sitter (RN-AdS$_4$) black holes with various topologies by charged scalar field
perturbations. We numerically find that the RN-AdS$_4$ black holes become
unstable against the linear perturbations below a critical temperature. It is
analytically shown that charge extraction from the black holes occurs during
the unstable evolution. To explore the end state of the instability, we
perturbatively construct static black hole solutions with the scalar hair near
the critical temperature. It is numerically found that the entropy of the
hairly black hole is always larger than the one of the unstable RN-AdS$_4$
black hole in the microcanonical ensemble. Our results support the speculation
that the black hole with charged scalar hair always appears as the final fate
of the instability of the RN-AdS$_4$ black hole.
