In this fourth and last paper on the biosystematics of the Dutch halophilous species of Spergularia, some relevant details concerning their reproduction and dissemination are discussed, the results of a karyogenetic investigation are reported, and, finally, the taxonomic interpretation of the acquired data is given. S. media and S. marina differ in their mode of reproduction; the latter normally reproduces autogamously in nature and the former both allogamously and autogamously. Dispersal of the seeds is by polychory. S. media and S. marina have 2n = 18 and 2n = 36 chromosomes respectively. Hybridisation apparently does not occur. The taxonomic confusion is shown to be chiefly caused by an insufficient insight into the degree of variation of certain characters, especially in the development of the seed wing, in both species.