Title: Reidemeister spectra for solvmanifolds in low dimensions,
Abstract: The Reidemeister number of an endomorphism of a group is the number of
twisted conjugacy classes determined by that endomorphism. The collection of
all Reidemeister numbers of all automorphisms of a group $G$ is called the
Reidemeister spectrum of $G$. In this paper, we determine the Reidemeister
spectra of all fundamental groups of solvmanifolds up to Hirsch length 4.