It is not known from very good fossil remains (only one skull) and there is a controversy should it be classified as a therapsid or not.
Tetraceratops actually does have six horn-like bumps in its skull. But when it was named the skull was still embedded in a matrix and all horns were not yet visible. Anyway it does have four horns on its snout and two large spines projecting from the corners of its jaw.
Tetraceratops DOES NOT have anything to do with ceratopsian dinosaurs like Triceratops other than it does have "horns". Tetraceratops is related to mammals and it lived millions of years before the time of the dinosaurs.