  If $S=(s_1,s_2,\ldots)$ is a non-decreasing sequence of positive integers,
then the $S$-packing $k$-coloring of a graph $G$ is a mapping $c:
V(G)\rightarrow[k]$ such that if $c(u)=c(v)=i$ for $u\neq v\in V(G)$, then
$d_G(u,v)>s_i$. The $S$-packing chromatic number of $G$ is the smallest integer
$k$ such that $G$ admits an $S$-packing $k$-coloring. A graph $G$ is
$\chi_S$-vertex-critical if $\chi_S(G-u) < \chi_S(G)$ for each $u\in V(G)$. If
$G$ is $\chi_S$-vertex-critical and $\chi_S(G) = k$, then $G$ is
$k$-$\chi_S$-vertex-critical. In this paper, $4$-$\chi_S$-vertex-critical
graphs are characterized for sequences $S = (1,s_2, s_3, \ldots)$ with $s_2 \ge
3$. There are $28$ sporadic examples and two infinite families of such graphs.
