Since the start of the 1990s, many of the earliest production tramcars have gone through extensive refurbishment and rebuilding, including the replacement of folding doors and the installation of low-floor center sections.

The first steps into the KT4s design were made when ČKD Tatra modified a six-axle K2 tramcar, to a four-axle suspended articulation formation which later presented itself in the KT4.

[3] The Soviet Union ordered the KT4SU for their meter-gauge tramways, the following cities received deliveries: The KT4YU is the Yugoslav variant of the tramcar, which were delivered to the Serbian and Croatian capitals.

Those tramcars were equipped with IGBT modules and recuperative braking and named KT4M-YUB (where B stands for Belgrade to be distinguished from Zagreb model).

A series of KT4 trams were also produced for Pyongyang, North Korea by the Shenyeng Passenger Vehicle Factory in China named ST4,[4] but have subsequently had their articulation removed due to structural defects of the joint.