Police in Xuan Que Commune said the group gathered on an overpass above the expressway after riding there on motorcycles.
According to investigators, the youths ate rambutan while watching traffic below, spitting seeds and dropping peels onto passing vehicles. One suspect allegedly threw a rambutan seed directly at the windscreen of a white car travelling at high speed.
The four have admitted their actions.
Although no injuries or major damage were reported, authorities said the incident posed a serious threat to road safety and showed signs of the offence of disturbing public order under Vietnam's Criminal Code. The investigation is continuing.
Police warned that even small objects, including fruit seeds, can become dangerous projectiles when vehicles are travelling at speeds of up to 120 kph on expressways, potentially causing serious accidents.
Authorities urged parents to educate young people about traffic safety and reminded the public not to stop on overpasses or throw any objects onto vehicles or roads.
