Normal subgroups generated by a single polynomial automorphism

We study criteria for deciding when the normal subgroup generated by a single polynomial automorphism of $\mathbb{A}^n$ is as large as possible, namely equal to the normal closure of the special linear group in the special automorphism group. In particular, we investigate $m$-triangular automorphisms, i.e. those that can be expressed as a product of affine automorphisms and $m$ triangular automorphisms. Over a field of characteristic zero, we show that every nontrivial $4$-triangular special automorphism generates the entire normal closure of the special linear group in the special tame subgroup, for any dimension $n \geq 2$. This generalizes a result of Furter and Lamy in dimension 2.