The first three volumes of the Encyclopaedia Aethiopica (vol. 1: A-C, 2003; vol. 2: D-Ha, 2005; vol. 3: He-O, 2007) are now being followed by volume 4, which includes 970 articles ranging alphabetically from O to W. The volume, written in English, comprises ca. 1150 pages and was compiled in cooperation with 280 authors coming from 26 countries. As with the preceding volumes, the geographic scope remains Ethiopia, Eritrea, Djibouti and Somalia; such regions as Sudan, Kenya, and the Arabian Peninsula are considered as well. The reference work's main focus is the humanities. The topics covered include basic elements of country study with fundamental geographic facts; topics from history, language and literature studies, art and culture, religion and ethnography, as well as aspects of social, political and economic development; and the region's relationships with Near East and the Western countries well into the second half of the 20th century. Key actors of regional history, politics and research are treated in biographic articles. All entries are supplied with bibliographic references with a distinction being made between primary and secondary sources. Illustrations and maps provide additional information where possible. Cross-references place entries in due thematic context. The Encyclopaedia Aethiopica will be completed by volume 5, which will include articles ranging from Y to Z and additional entries, along with overview tables, maps and a general index.