GT4 is a German acronym for an articulated (G for Gelenk = "joint") motorized (T for Triebwagen) tramcar with 4 axles. Such vehicles running on two bogies were developed in Germany, starting in 1959, in different places for different tram systems (in Stuttgart and Bremen), and 15 years later by ČKD Tatra in Czechoslovakia as KT4 ("KT" representing kloubová tramvaj, from kloub = "joint", in Czech and Slovak).
References
GT4 (tram) Wikipedia(Text) CC BY-SA