A planar graph is rectilinear planar if it admits a planar orthogonal drawing without bends. For series-parallel graphs in the variable embedding setting the best known testing algorithm takes $O(n^2)$ time. We show that rectilinear planarity of a series-parallel graph with maximum degree four can be tested in $2^{O(k)}\cdot n$ time, where $k$ is the number of degree-4 vertices; the planar embedding is not fixed. In particular, the running time is subquadratic when $k=o(\log n)$. Each node of the SPQ$^*$-tree stores the set of its admissible spiralities; we show that this set is a union of $2^{O(k)}$ arithmetic progressions of step two. All proofs are in the paper and are additionally verified in Lean 4 with Mathlib.

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