 For mass numbers with more than two beta-stable isobars, quadruple beta decay and its inverse, quadruple electron capture, have been proposed as alternatives to double beta decay in the isobars with the greatest energy excess.These decays are energetically possible in eight nuclei, though partial half-lives compared to single or double beta decay are predicted to be very long; hence, quadruple beta decay is unlikely to be observed.The eight candidate nuclei for quadruple beta decay include 96Zr, 136Xe, and 150Nd capable of quadruple beta-minus decay, and 124Xe, 130Ba, 148Gd, and 154Dy capable of quadruple beta-plus decay or electron capture.