The effect of deformation-induced disorientations on work-hardening of metals is modelled by dislocation dynamics. By incorporating excess dislocations related to disorientations, Kocks' dislocation model describing stage III hardening is extended to stage IV. Disorientations evolving from purely statistical reasons still lead to stage III behaviour and a saturation of the flow stress, but deterministic contributions to the development of disorientations, as differences in activated slip systems across boundaries, cause a linear increase of the flow stress at large strains. Such a constant work-hardening rate is characteristic for stage IV.