The four suspects are: a man who was detained at the Stansted Airport on Friday on suspicion of manslaughter and conspiracy to traffic people; a couple taken into custody from Warrington on the same charges; lorry driver Mo Robinson held on suspicion of murder since Wednesday, the BBC reported.
Meanwhile, police have been given extra time to question Robinson of County Armagh.
The arrests came as efforts to remove and identify the bodies of eight women and 31 men, who were found in the refrigeration trailer at an industrial park in Grays, continued.
It is believed that a number of the people were from Vietnam, despite Essex Police initially saying the victims were all Chinese nationals.
And VietHome, an organisation that represents the Vietnamese community in the UK, said it had received photos of nearly 20 people reported missing, since the lorry was discovered.
The group said it began to receive reports of missing people on Wednesday. Those who have disappeared were aged between 15 and 45.
The BBC spoke to the brother of Pham Thi Tra My, 26, who has not been heard from since she sent text messages at 10.30 p.m. on Tuesday saying she could not breathe two hours before the trailer arrived at the Purfleet terminal from Zeebrugge in Belgium.
Her family have shared texts she sent to her parents which read: "I am really, really sorry, Mum and Dad, my trip to a foreign land has failed. I am dying, I can't breathe..."
Relatives of Nguyen Dinh Luong, 20, have also said they fear he is among the 39 victims.
The Vietnamese Embassy in London has been in contact with Essex police since Thursday, a spokesman confirmed.
Liu Xiaoming, the Chinese Ambassador to the UK, said China was taking "firm and effective measures against human trafficking".
The trailer arrived in Purfleet on the River Thames from Zeebrugge in Belgium at 12.30 a.m. on Wednesday.
The lorry and trailer left the port at Purfleet shortly after 1.05 a.m. the same day.
Ambulance staff discovered the bodies in the container at Waterglade Industrial Park in Grays at about 1.30 a.m.
Vigils were held outside the Home Office in London and at the front of City Hall in Belfast on Thursday.
( With inputs from IANS )