The term Fourth World first came into use in 1974 with the publication of Shuswap Chief George Manuel's THE WORLD IN INDIAN REALITY ( Amazon link to the book), the term refers to nations Cultural entities, ethnic groups) of indigen living within or across state boundaries (nations state). This is in reference to American Indians.
The use of the term three worlds is outdated. Now some people say there are four world's in this world,
1. First World: The bloc of emocratic industrial developed countries within the American influence.
2. The Eastern Bloc: consists of Communist -socialist states under the influence of USSR,
3. The remaining three quarters of the world's population , aligned to neither of the above two blocs,
4. The term "Fourth World", coined in the early 1970's by Shuswap Chief George Manuel, refers to widely unknown nations (cultural entities) of indegeneous people.
N.B. Some of the very poorest countries, espcially in Africa, the countries having no indusrialisation and with almost agrarian status and no hope of competing world market are sometimes termed as Fourth World.