Rescuers are spending the night searching for victims trapped beneath the buckled structure.
The 862-tonne, 174ft (53m) bridge fell over an eight-lane motorway on Thursday afternoon, crushing at least eight vehicles, police said.
The bridge was erected on Saturday in just six hours.
It was built using a method called “accelerated bridge construction” to avoid traffic disruption. A major section of the bridge was assembled on the side of the road and then raised into place.
Miami-Dade Fire Chief Dave Downey confirmed on Thursday night that at least four people were killed.
“This is going to be a long incident. We are going to have rescuers here through the night, if not longer,” Chief Downey said.
“We’re working our way into the pile trying to create holes that we can actually physically see,” Miami-Dade Fire Department Division Chief Paul Estopian told reporters.
At least 10 people were treated for injuries at the Kendall Regional Medical Center, including two who were in “extremely critical condition”, according to Dr Mark McKenney, director of general surgery.
At one point, police ordered TV helicopters to leave the area so rescue teams could hear for any sounds of people calling for help under the wreckage, CBS Miami television reported.
FIU students are currently on their spring break until 17 March. (Courtesy BBC)