  We study the multivariate independence polynomials of graphs and the
log-concavity of the coefficients of their univariate restrictions. Let
$R_{W_4}$ be the operator defined on simple and undirected graphs which
replaces each edge with a caterpillar of size $4$. We prove that all graphs in
the image of $R_{W_4}$ are what we call pre-Lorentzian, that is, their
multivariate independence polynomial becomes Lorentzian after appropriate
manipulations. In particular, as pre-Lorentzian graphs have log-concave (and
therefore unimodal) independence sequences, our result makes progress on a
conjecture of Alavi, Malde, Schwenk and Erd\H{o}s which asks if the
independence sequence of trees or forests is unimodal.
