With a length of 230 km (140 mi),[1] it connects the capital of Phnom Penh with Sihanoukville in the south-west.

[2] Sihanoukville is the only international sea port of Cambodia, making NH4 one of the country's most important highways.

[3] It is also considered the most dangerous road in Cambodia due to the occurrence of several traffic accidents and limited management by authorities.

The junction with NH3, which leads to Kampot to the east beyond to the Vietnamese border is in the territory of Sihanoukville, at Prey Nob District.

The expressway is built through Chinese investment at a cost of $2 billion under a build-operate-transfer contract by China Road and Bridge Corporation.