In this paper we present the results of a systematic comparison between the values of the bond lengths, bond angles, and vibrational frequencies of group IV dihydrides and tetrahydrides (XH(2), XH(4), X=Si,Ge,Sn,Pb), and monoxides (XO, X=Ge,Sn,Pb), as well as the XH(4)-->XH(2)+H-2 reaction energies, calculated with the spin-free (Cowan-Griffin based) quasirelativistic ab initio core model potential method (AIMP) and the all-electron Dirac-Hartree-Fock method (DHF), using basis sets of similar quality in their valence part. The deviations between the AIMP and DHF results on the absolute values of the properties and on the sizes of the relativistic effects are of the order expected for an effective core potential method and follow the expected tendency of importance of the spin-orbit effects. The quality of the AIMP results is shown to be consistent going down the group IV of the Periodic Table.