The storm's first casualties included a mother and her baby, who died when a tree fell on their brick house in Wilmington, North Carolina. The father was injured in the accident.
In Washington, North Carolina, the wind-swept Pamlico River burst its banks and flooded entire neighbourhoods. A fourth victim was killed in Lenoir County while plugging in a generator, according to the governor's office.
In New Bern, North Carolina, the storm surge "overwhelmed" the town, located at the confluence of the Neuse and Trent rivers, Cooper said.
Florence had been a Category 3 hurricane on the five-step Saffir-Simpson scale with 120-mph winds as of Thursday, but dropped to a Category 1 hurricane before coming ashore.
Florence's forward movement during the day slowed to a near-standstill - sometimes it was going no faster than a human can walk - and that enabled it to pile on the rain.
The National Weather Service is predicting that the hurricane will blow ashore as early as Friday afternoon along the North Carolina-South Carolina line, then push its rainy way westward, with a potential for catastrophic inland flooding.
But forecasters said its extreme size meant it could batter the US East Coast with hurricane-force winds for almost a full day.
At least 1.7 million people in the Carolinas and Virginia have been warned to evacuate, and others are hunkering down. Numerous evacuees took their pets.
"But I'm staying", she said.
Cooper said Florence was expected to cover nearly all of North Carolina in several feet of water.
Screaming winds bent trees and led to near-horizontal rain as Florence's leading edge whipped the Carolina coast to begin an onslaught that could last for days, leaving a wide area underwater from both heavy downpours and rising seas.More news: Colbert fact-checks Trump's 'sickening' Hurricane Maria comments
Hurricane Florence made landfall near Wrightsville Beach, North Carolina at 7:15 AM EDT (1115 UTC) with estimated maximum winds of 90 miles per hour (150 km/h), and a minimum central pressure estimate of 958 millibars.
"While Florence's maximum winds are expected to weaken a little, it is still expected to remain a risky major hurricane as it approaches the coast", they warned in a Wednesday mid-day update.
New Bern Police Lt. David Daniels said that between 150 and 200 people were rescued so far, with others still waiting for help.
"These are folks who chose to stay and ride out the storm for whatever reason, despite having a mandatory evacuation", city public information officer Colleen Roberts said. "These are folks who are maybe in one-story buildings and they're seeing the floodwaters rise".
Their graphics look like something out of a video game, featuring a reporter in the center of a screen and water towering overhead to show how high the water can get. "We're able", Trump said Wednesday outside the White House.
The National Hurricane Center said Florence will eventually break up over the southern Appalachians and make a right hook to the northeast, its rainy remnants moving into the mid-Atlantic states and New England by the middle of next week.
"It's insane", he said in a phone interview. "I've never seen tree devastation this bad".
At 8 a.m. EDT, NHC noted "A life-threatening storm surge is already occurring along portions of the North Carolina coast and will continue through today and tonight".
Utility officials have warned that widespread power outages are likely and that it could take weeks to restore electricity. After a Friday landfall near Cape Fear, N.C., the storm is expected to drift slowly west, battering the Carolinas through the weekend.
About 10 million people could be affected by the storm. Unfortunately, sweet potatoes and peanuts are just getting underway with harvests, and I'll remind you that we're number one in the nation in sweet potatoes.