The SS4 is a member of the third generation of electric locomotives built in China. Built for heavy freight service due to growing demands in the eighties, it is a two-section locomotive with a total of eight axles. It delivers 6,400 kW and has a starting tractive effort of 628 kN, at a top speed of 100 km/h. Three prototypes were designed and built by Zhuzhou in 1985 and production was started in 1989. After 158 of the original variant, production switched to the improved SS4G. This incorporated a more modern electrical part that used advancements from the imported 6K and 8K locomotives. A total of 1,474 SS4G were built at Zhuzhou, Datong, Ziyang and Dalian.

The SS4B is a variant with completely overhauled electrical part built between 1995 and 2010. Due to their higher price, the 260 locomotives were only used by Shenhua Group for heavy coal traffic. Finally, there is the SS4C of which only two were built in 1997. These have an axle load of 25 instead of 23 tonnes to increase traction on freight trains of 5,000 tonnes. But no further locomotives of this variant were built since the increased axle load alone did not substantially improve haulage capacity.