The group-IV monochalcogenides SnS, SnSe, GeS, and GeSe form a family within the wider group of semiconductor 'phosphorene analogues'. Here, we used first principles calculations to investigate systematically their structural, electronic and optical properties, analyzing the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. We show that all those binary phosphorene analogues are semiconducting, with bandgap energies covering part of the infra-red and visible range, and in most cases higher than phosphorene. Further, we found that they have multiple valleys in the valence and conduction band, the latter with spin-orbit splitting of the order of 19-86 meV.