The small waterstriders of the subfamily Trepobatinae have radiated extensively on New Guinea and in surrounding archipelagos. All of the marine forms of the subfamily are found in this region, with the exception of one monotypic genus occurring on brackish water in the eastern tropical Pacific. The present study, the fourth in a series of reports dealing with Melanesian Trepobatinae, covers these regional marine taxa. The tribe Stenobatini, proposed in Part 1 of this series to hold the genera Stenobates Esaki, type-genus, plus Rheumatometroides Hungerford and Matsuda, and Stenobatopsis gen. n. (as undescribed genus 2), is revised, two new genera are proposed, and keys to genera and species are provided, followed by synonymies, diagnoses and discussion of the constituent genera. The following new taxa are proposed within the Stenobatini: Pseudohalobates gen. n., monobasic, type-species S. inobonto sp. n. from Indonesia (Celebes, Moluccas, Talaud Archipelago, Biak and Yapen islands, and Vogelkop Peninsula of Irian Jaya), and the Philippines (southern Mindanao); Stenobatopsis gen. n., monobasic, type-species S. stygius sp. n. from Halmahera; Thetibates gen. n., type-species Rheumatometroides serena Lansbury, from northern Papua New Guinea and the Solomon Islands; Rheumatometroides kikori sp. n. from the Kikori delta of southern Papua New Guinea; R. insularis papar n. ssp. from Sabah, north Borneo; R. sele sp. n. from the Vogelkop Peninsula of Irian Jaya; R. wabon sp. n. from Biak Island; Stenobates fakfak sp. n. from the Vogelkop Peninsula of Irian Jaya; S. kamojo sp. n. from Biak Island, Salawati Island, Yapen Island, and the Vogelkop Peninsula of Irian Jaya; S. kasim sp. n. from Salawati Island; S. labuha sp. n. from Bacan and Halmahera; S. langoban sp. n. from Palawan; S. sangihe sp. n. from the Sangihe Archipelago; and S. zamboanga sp. n. from Mindanao. Other nomenclatural changes: Rheumatometroides carpentaria (Polhemus & Polhemus), comb. n.; Rheumatometroides insularis insularis (J. Polhemus & Cheng), comb. n.; Stenobates makraitos (Chen & Nieser) comb. n.; Thetibates matawa (Lansbury), comb. n. [ = Rheumatometroides aqaaqa (Lansbury), syn. n.]; Thetibates serena (Lansbury), comb. n. Habitat and distributional data are given for these taxa, accompanied by keys, figures of key characters and distribution maps.