  The unusual magnetic and electronic ground states of 5d iridates has been
shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The
influence of appreciable but reduced SOC in creating the manifested magnetic
insulating states in 4d oxides is less clear, with one hurdle being the
existence of such compounds. Here we present experimental and theoretical
results on Sr$_4$RhO$_6$ that reveal SOC dominated behavior. Neutron
measurements show the octahedra are both spatially separated and locally ideal,
making the electronic ground state susceptible to alterations by SOC. Magnetic
ordering is observed with a similar structure to an analogous
${J}_{\mathrm{eff}}\mathbf{=}1/2$ Mott iridate. We consider the underlying role
of SOC in this rhodate with density functional theory and x-ray absorption
spectroscopy and find a magnetic insulating ground state with
${J}_{\mathrm{eff}}\mathbf{=}1/2$ character.
