  The $\textit{planar slope number}$ $psn(G)$ of a planar graph $G$ is the
minimum number of edge slopes in a planar straight-line drawing of $G$. It is
known that $psn(G) \in O(c^\Delta)$ for every planar graph $G$ of maximum
degree $\Delta$. This upper bound has been improved to $O(\Delta^5)$ if $G$ has
treewidth three, and to $O(\Delta)$ if $G$ has treewidth two. In this paper we
prove $psn(G) \leq \max\{4,\Delta\}$ when $G$ is a Halin graph, and thus has
treewidth three. Furthermore, we present the first polynomial upper bound on
the planar slope number for a family of graphs having treewidth four. Namely we
show that $O(\Delta^2)$ slopes suffice for nested pseudotrees.
