Officials were still assessing the full extent of property damage on Wednesday in Greenfield, a community of 100km southwest of Des Moines, the state capital.
The tally of casualties was announced by the Iowa Department of Public Safety at the end of a day in which rescue teams picked through fields of debris left by the deadly twister in search of people who might be trapped in the wreckage.
"When we have this many homes that have been destroyed, fully demolished, we want to make sure every person is accounted for," Iowa State Patrol Sergeant Alex Dinkla told a morning news conference.
Local residents clean up debris from tornado damaged homes in Greenfield, Iowa. (AP PHOTO)
The agency did not immediately say whether anyone was still reported missing or whether the search for victims was continuing.
No details were provided about the precise circumstances of the four storm-related fatalities. Their identities are being withheld pending notification of next of kin.
Of the 35 people reported injured by the tornado, at least 14 were taken to out-of-county hospitals for medical treatment, officials said, adding the actual number of people hurt was most likely higher.
Images from Greenfield showed a wide swath of utter destruction, with homes reduced to splinters, trees uprooted, vehicles crushed and debris strewn everywhere. Several large wind turbines outside of town had been toppled or snapped in two.
"It's horrific. It's hard to describe," said Iowa Governor Kim Reynolds, who declared a disaster emergency for 15 counties.
The twister that touched down in Greenfield was among a swarm of tornadoes reported in southwestern Iowa on Tuesday afternoon and evening. The Federal Emergency Management Agency said preliminary reports counted 23 potential tornadoes, mostly in Iowa.
Reynolds said state officials were working to send a request for President Joe Biden to approve a disaster declaration to get federal assistance for state residents.
Among the buildings damaged in Greenfield was a hospital, forcing authorities to create a makeshift medical care centre at the lumberyard and send some of the injured to other area facilities.
State Representative Ray Sorensen said several residents used their own vehicles to transport those who were injured to safety moments after the storm struck.
"We pulled a guy from the rubble and put him on a little makeshift stretcher we made and threw him in the back of a truck," he said.
Severe thunderstorms packing large hail and damaging winds were forecast again on Wednesday across the Southern Plains, with tornado watches posted for much of Texas, as well as parts of Oklahoma, Arkansas, Louisiana, Mississippi and Tennessee.
A separate storm system threatened to spawn tornadoes in parts of New York, Pennsylvania, Maryland and Virginia, according to the National Weather Service.
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