Two new species of the family Amaurobiidae are described from the Ryukyu Islands, southwestern Japan, under the names Callobius cavernarius sp. n. (Okinoerabu Island) and Callobius shimojanai sp. n. (Okinawajima Island). Both species are closely related to each other and there are three further known species related to the new species: C. yakushimensis Okumura 2010, C. amamiensis Okumura et al., 2018 and C. breviprocessus Okumura et al., 2020. The morphological differences between these five species are discussed in detail, especially those in the structure of the male palpal organ and the female genitalia. Although this species group includes cavernicolous species, no particular adaptations to living in caves, for example degeneration of the eyes, was observed. A map showing the distribution of these species is given.

The crabs collected by the RV Hakuhō Maru (KH-72-1 cruise) in the South China Sea are recorded as the fourth part following the previous three parts of the series (Sulu Sea and Sibutu Passage, Timor Sea, and Sahul Shelf). They are mostly shallow water inhabitants, representing 35 species of 31 genera in 13 families, with two new species of the family Epialtidae, Naxioides elongatus sp. nov. and Samadinia hakuhoae sp. nov. Taxonomic comments are given for each species. In this fourth and final part, biogeographic notes on all the species collected by RV Hakuhō Maru (KH-72-1 cruise) are briefly summarized, together with a list of the crabs collected by the RV Hakuhō Maru (KH-73-2 cruise) in the South China Sea.

Beleses eikoae Togashi, 2004 is synonymized with Beleses nigrifemoratus Togashi, 1999. Additional descriptions are given for Beleses nigrifemoratus, Caliroa vaccini Okutani, 1965 and Fagineura parva Hara, 2022. The males of Beleses nigrifemoratus and Euura damnacanti (Takeuchi, 1922) are described for the first time. Abia triangularis (Takeuchi, 1931) is recorded from Hokkaido for the first time. New host plant records are given for Aproceros leucopoda Takeuchi, 1939, Arge gracilicornis (Klug, 1814), Beleses nigrifemoratus, Euura damnacanti, Fagineura parva, Heterarthrus alnivorus Togashi, 1992, Mesoneura macroptera Takeuchi, 1936, Mesoneura mikagei Togashi, 1998, Tomostethus nigritus (Fabricius, 1804) and Monoxiphia harai (Shinohara, 2019). Notes on life history, larvae or distribution are given for Arge gracilicornis, Pamphilius kamikochensis Takeuchi, 1930, Ametastegia polygoni Takeuchi, 1929, Beleses nigrifemoratus, Caliroa vaccini, Euura damnacanti, Fagineura parva, Mesoneura macroptera, Mesoneura mikagei, Tomostethus nigritus, Xiphydria kanba Shinohara, Hara and Smith, 2020, and Xyelecia japonica Togashi, 1972.

Morphological characters and distribution of the rare deep-sea zoarcid fish, Lycenchelys maculata, are reviewed, including osteology and coloration. Eleven additional specimens document the species distribution in the Pacific Ocean from off Simushir Island (Kuril Islands) to off Kakegawa (Shizuoka Prefecture, Japan) at depths of 200–489 m. A photograph of a fresh specimen (32 cm TL) reported to be the species, published in 1984 and 1988, is not in part of the type specimens. Intraspecific variations are also reported for the color of tail and the pelvic fins. Diagnostic characters are revised and the taxonomic history and associated problems provided.

A single specimen (14.9 mm in standard length) of Trimma panemorfum Winterbottom and Pyle, 2022, a species recently described on the basis of three specimens from deep coral reefs off Palau, was collected from Okinawa Island, Ryukyu Islands, southern Japan. A description of the Japanese specimen, the first Japanese record of T. panemorfum, is provided, and the new standard Japanese name “Kosumosu-benihaze” proposed for the species.

Balaenopterid whale specimens preserved in several institutions totaling 19 were examined. Specimens identified to species level were 14, i.e. 3 Balaenoptera musculus, 6 B. edeni, and 5 B. omurai. The remaining 5 specimens were not identified. The existence of B. omurai from Myanmar waters was confirmed based on the morphological characters of the skull and the first rib. This article is the first confirmation of this species in Myanmar. Among the two already “confirmed” species, B. musculus was identified by Smith et al. (1997) based on the actual specimen in the Natural History Museum, however for B. edeni, the specimens we examined were not observed for species identification and no preceding work reported the existence of B. edeni, except for the type specimen collected in Mon state in 1871 and now preserved in the Indian Museum, Kolkata (Anderson, 1879).