The dead included a mother and baby who were killed when a tree fell on their home in Wilmington, North Carolina.
Forecasters warned that drenching rains of 30 centimetres to 1 metre as the hurricane-turned-tropical storm crawls westward across North and SC could trigger epic flooding well inland over the next few days.
The father was injured in the incident and was transported to a hospital, officials said.
Two other people died in North Carolina's Carteret County, according to the county's emergency management services office.
Tropical storm conditions will continue along the coast within the tropical storm warning area and also well inland across portions of SC and North Carolina Saturday.
Since making landfall at Wrightsville Beach in North Carolina on Friday morning, Florence's forward speed has slowed down dramatically, with the storms center of circulation only about 100 miles away from Wrightsville Beach a full 24 hours after landfall. Duke Energy expects between 1 million to 3 million homes and businesses to lose power.
Rainfall has been heavy and consistent in the areas of the storm, with Florence expected to cascade up to 40 inches of rain across southern and central portions of North Carolina into far northeastern SC. The deaths were among the first recorded fatalities from the storm, which also killed a 77-year-old man who was knocked over by wind after going outside to check on his dogs.
"It will not stop, this weather event of Florence", Begnaud said.
The Navy says nearly 30 Virginia-based ships and 128 aircraft sent away from their bases in the Hampton Roads-area because of now-Tropical Storm Florence have been given the go-ahead to return.
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They added that the onslaught would last for hours because Florence was barely creeping along at 9km an hour and still drawing energy from the ocean. Electricity remained out for much of the city, with power lines lying across many roads like wet strands of spaghetti. CORRECTING GRAMMAR AND TYPO IN CITY NAME People are rescued by members of the U.S. Army during the passing of Hurricane Florence in the town of New Bern, North Carolina, U.S., September 14, 2018.
By Saturday morning, top sustained winds had weakened to 50 mph as it moved farther inland at 2 mph about 35 miles west of Myrtle Beach, South Carolina.
"Florence is here to stay for awhile", McMaster said from Columbia Friday afternoon.
"I feel like the dumbest human being who ever walked the face of the earth", he said.
"The cost to SC, if that happens, if the storm continues the way it is going, could be in the billions", Dahler said. Forecasters are predicting that the Lumber and Cape Fear rivers will crest significantly higher than after Hurricane Matthew, and in some areas, Florence will bring 1,000-year rainfall totals, according to the governor.
"WE ARE COMING TO GET YOU", New Bern city officials said on Twitter.
Governor Cooper said there have been "several hundred" rescue operations and "there are still some people they need to get to".
For people living inland in the Carolinas, the moment of maximum peril from flash flooding could arrive days later, because it takes time for rainwater to drain into rivers and for those streams to crest.
Florence will continue to move very slowly to the southwest on Saturday, which in turn will drag the axis of heavy rain bands to the southwest as well.