This paper is the fourth in a series revising the taxonomy of New Zealand dictyoceratid sponges (phylum Porifera, class Demospongiae, order Dictyoceratida). In the course of this work the thorectid sponge genus Cacospongia Schmidt is redefined and amended to provide a less ambiguous taxon, and to more closely reflect the morphology of the type species Cacospongia mollior Schmidt, 1862. A new genus, Scalarispongia n.gen. , is established for sponges previously assigned to Cacospongia, with Cacospongia scalaris Schmidt, 1862 designated as the type species. A third genus, Semitaspongia n.gen. , is proposed to accommodate another group of sponges which are similar to C. scalaris Schmidt, but which display consistent morphological differences in their fibre skeleton, and in the deposition of collagen within the mesohyl. Five undescribed species from New Zealand, previously assumed to belong to Cacospongia, have been assigned to the genus Semitaspongia: Semitaspongia incompta n.sp. , S. glebosa n.sp. , S. nigracorda n.sp. , S. pulvinata n.sp. and S. bactriana n.sp. The designated type species is Semitaspongia incompta. One species in this group of genera has previously been reported from New Zealand, and this is referred to Cacospongia.