Customs and Border Protection officers stopped a 2014 Chrysler 200 that was attempting to cross at San Ysidro on Tuesday when a CBP dog alerted to the vehicle.
Officers opened the trunk and found three men and a woman stuffed inside. They were Chinese nationals who did not have legal permission to enter the United States, officials said.
The driver of the vehicle, a 24-year-old male U.S. citizen, is expected to face charges related to what officials call a human-smuggling attempt.
"Concealing persons in vehicles is dangerous and could have severe consequences," said Pete Flores, CBP's director of field operations in San Diego. "San Ysidro CBP officers stopped a violation of our immigration law and were able to resolve the incident safely."