Regular description for $\{a^n\mid n\in D\}$, where $D$ is the set of distances of paths (with allowed repetition of nodes in the path)
from node $0$ to node $4$ in the directed graph with edges labelled with lengths whose set of nodes is $\{0,1,2,3,4\}$ and whose set of edges is
$\{0\xrightarrow{4}1,\;1\xrightarrow{4}2,\;1\xrightarrow{6}3,\;2\xrightarrow{4}0,\;2\xrightarrow{2}4,\;3\xrightarrow{2}0,\;4\xrightarrow{4}3\}$