A Type IV civilization was a classification of technology advancement of a civilization.
It was based on the long standing theory of technological advancement which was not measured by specific inventions but rather on the use of energy. Type I civilizations were capable of controlling the energy of their own world whilst Type II civilizations were able to control the energy of entire solar systems and Type III controlled the power of an entire galaxy. Technically, thats where the theory ended with a Type III civilization.
In 2378, Spock suggested that they create a Type IV civilization category after witnessing the Vehicle and believed that its technological capabilities exceeded that of the lower level categories. It was theorized that such a civilization was capable of harnessing as well as manipulating the power along with the energy of the universe itself. (TNG novel: Forever Dark)