The nuclear structure systematics of superheavy nuclei lying at the extremes, namely Z=116, 118, 120, and 122, and their α-decay half-lives are investigated using relativistic mean-field (RMF) model, relativistic Hartree-Bogoliubov (RHB) theory, and nonrelativistic Hartree-Fock-Bogoliubov (HFB) method. In this study, we predict the half-life for unobserved elements consistently with the study of experimentally observed even-even superheavy nuclei with Z=106-118. The structural properties of considered nuclei are studied within the same theoretical models with different nuclear effective interactions. The calculated mass the defect has been used as input in the shape parametrization model of alpha decay to determine the α-decay half-lives. These calculations suggest significant uncertainty due to the sharp sensitivity of half-lives on the Qα values, although the ground-state observables agree fairly well.