The term 4th World was coined by Manuel Castells to refer to black holes of social exclusion. 4th World is deliberatively opposed to 1st World, 2nd World and 3rd World, all of which imply a certain degree of connectivity. This connectivity is what is notably absent in the notion 4th World.
Examples of places described as 4th World include Sub-Saharan Africa, rural Latin America and innercity ghettos or banlieus. Because of their disconnection from the rest of the world these places are powerless, and thus unable to change. In the case of ghettos or banlieus this disconnection stands in stark contrast with the city centres which are the most connected spaces.