  Inspired by the Odlyzko root discriminant and Golod--Shafarevich $p$-group
bounds, Martinet (1978) asked whether an imaginary quadratic number field
$K/\mathbb{Q}$ must always have an infinite Hilbert $2$-class field tower when
the class group of $K$ has $2$-rank $4$, or equivalently when the discriminant
of $K$ has $5$ prime factors. No negative results are known. Benjamin (2001,
2002) and Sueyoshi (2004, 2009, 2010) systematically established infinite
$2$-towers for many $K$ in question, by casework on the associated R\'{e}dei
matrices. Others, notably Mouhib (2010), have also made progress, but still
many cases remain open, especially when the the class group of $K$ has small
$4$-rank.
  Recently, Benjamin (2015) made partial progress on several of these open
matrices when the class group of $K$ has $4$-rank $1$ or $2$. In this paper, we
partially address many open cases when the $4$-rank is $0$ or $2$,
affirmatively answering some questions of Benjamin. We then investigate
barriers to our methods and ask an extension question (of independent interest)
in this direction. Finally, we suggest places where speculative refinements of
Golod--Shafarevich or group classification methods might overcome the `near
miss' inadequacies in current methods.
