The fourth volume of this series “Acarorum Catalogus” is devoted to the superfamilies Myobioidea (fam. Myobiidae) and Pterygosomatoidea (fam. Pterygosomatidae). About 693 sp. are listed therein, with synonymies and citations in various papers. The book also contains a list of by-country distributions and lists of authors who published on any known species of Myobiidae and Pterygosomatidae. There are also lists including hosts of these mites. The bibliography consists of more than 580 titles. Other volumes of the same series on the patterns of the first three are under way.