  This paper concerns the enumeration of isomorphism classes of modules of a
polynomial algebra in several variables over a finite field. This is the same
as the classification of commuting tuples of matrices over a finite field up to
simultaneous similarity. Let $c_{n,k}(q)$ denote the number of isomorphism
classes of $n$-dimensional $\mathbb{F}_q[x_1,\dotsc,x_k]$-modules. The
generating function $\sum_k c_{n,k}(q)t^k$ is a rational function. We compute
this function for $n\leq 4$. We find that its coefficients are polynomial
functions in $q$ with non-negative integer coefficients.
