According to police they took hostage driver and co-driver of the truck and demanded ransom with the truck's owner.
Of the four involved in the case, police have arrested only two men so far.
The arrested are Shankar Adhikari, of Hetaunda-1 and Bhuyat Thapa of Hetaunda-28.
Police said they had taken captive driver Ramesh Tokar of Dungrekhola-3 and co-driver Sunil Gautam of Chandranigahapur-8 at Hetauda, Lachka, while the truck was heading to Sarlahi from Nepalgunj.
They had demanded Rs 40,000 ransom with the truck owner after taking the truck 50 meters away from the highway, said DSP Shishu Sharma of District Police Office, Makawanpur.