  The gravitational wave source GW190412 is a binary black hole (BBH) merger
with three unique properties: i) its mass ratio is about 0.28, the lowest found
so far, ii) it has a relatively high positive effective spin parameter
\chi_eff=0.25, and iii) it is observed to be precessing due to in-plane
projected spin of the binary with an in-plane precession parameter \chi_p=0.3.
The two main formation channels of BBH formation fail to account for GW190412:
field formation scenarios cannot explain the observed precession unless by
invoking large natal kicks, and dynamical assembly in dense stellar systems is
inefficient in producing such low mass-ratio BBH mergers. Here, we investigate
whether "double mergers" in wide hierarchical quadruple systems in the "3+1"
configuration could explain the unique properties of GW190412. In this
scenario, a compact object quadruple system experiences two mergers: first, two
compact objects in the innermost orbit merge due to secular chaotic evolution.
At a later time, the merged compact object coalesces with another compact
object due to secular Lidov-Kozai oscillations. We find that our scenario is
consistent with GW190412. In particular, we find a preferential projected spin
around \chi_p=0.2. However, the likelihood of a double merger is small and the
formation efficiency of these systems is uncertain. If GW190412 originated from
a double merger in a 3+1 quadruple, we find a strong constraint that the first
merger likely occurred between roughly equal-mass BHs in the innermost orbit,
since the recoil velocity from unequal-mass BHs would otherwise have disrupted
the system.
