The bodies of 59 men, eight women and four children have reportedly been found in the lorry.
The victims include a girl thought to be aged between one and two years old, and three boys aged between eight and 10, according to news agency Reuters.
They are thought to have been dead for nearly two days.
Police have said the bodies appeared to be those of Syrian migrants who probably died from suffocation, the BBC reports.
Local police have said that three of the people arrested are Bulgarian and one is Afghan.
They are being detained in Hungary.
Forensic teams worked throughout the night to examine the corpses at a customs building with refrigeration facilities, where the truck was towed to.
The corpses were found in the back of the refrigerated food delivery truck, which had been parked on the hard shoulder of the A4 road, Austria's “Eastern Motorway”, near the town of Parndorf.
An Austrian police motorway patrol officer had seen fluids seeping from its back door.
A police manhunt has been launched to find the driver of the lorry, who is suspected of smuggling the victims into the country.
Police said the people may have already been dead when the lorry entered the country from Hungary the previous night.
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