We evaluate the flavor-changing neutral-current decays {ital S}({ital v{bar v}}){r arrow}{ital c{bar t}},{ital t{bar c}},{ital a{bar t}},{ital t{bar a}} of the {ital S}-wave quarkonium states formed from the {ital Q}={minus}1/3 charge {ital v} quark of a fourth-generation quark doublet ({ital a},{ital v}). Under favorable mass and mixing conditions these decay modes could occur with branching fractions of order 1%.