Translational energy spectra have been obtained for single-electron capture by 8 keV Ar4+ ions in He, Ne and Ar. The spectra for He and Ne targets display eight well-resolved peaks, the majority of which correspond to capture involving a rearrangement of the electron core configuration. By contrast, the five channels observed in the spectrum for the Ar target are all associated with core-conserved electron-capture processes. One pronounced peak in the spectra for He and Ne cannot be explained on the basis of capture from the ground 3P or metastable 1D and 1S states of the Ar4+ ions, and appears to originate from capture by Ar4+ in the hitherto unidentified metastable 5S0-state at ∼ 7.5 eV above the ground state. All observed channels are spin-conserved.