She had been playing on the trampoline in a section of Gorleston beach called Bounce About, which also has an inflatable slide, traditional trampolines and a rodeo ride.
Owner Curt Johnson, 47, who was not present at the time of the tragedy, said the inflatable may have exploded in the heat.
Temperatures reached 23C at the tourist attraction, 5km from Great Yarmouth, as families made the most of the heatwave.
"It is very upsetting," Mr Johnson added. "We have been at the beach for a very long time, definitely a number of years and it is the first accident we have had there and it is quite shocking."
One witness said: "Just seen the most horrific thing in my life.
"A bouncy castle exploded at the beach and the child on it was catapulted about 20ft into the air.
"PLEASE do not allow your children on a bouncy castle in this heat and PLEASE say a prayer for the 4 year old [who was] rushed to hospital after 15 minutes of CPR.
"Totally heartbroken to hear that the little girl didn't make it. Thoughts and prayers go out to her family."
Karen Sell, who was leaving the beach at the time, said: "I was walking away from the beach and I heard the most enormous bang and straight away turned round.
"I was at the top of the stairs near the cliff that overlooks the beach and people were all there and everyone was in shock.
"I spoke to someone who described how the girl was just thrown about 20ft into the air and landed on the floor.
"It was just terrible. A man was giving the girl CPR for quite a while before they took her hospital.
"The bouncy castle was not like one I have seen before. It was like a trampoline so you could jump a lot higher on it than a normal one."
Kayla-Ann Weaver, who was sitting only metres away from the accident, said: "I cannot believe what I just witnessed on Gorleston beach. Never ever will I let my girls on a bouncy castle ever. My thoughts are with the little girls family.
"I was right next to it the girls wanted to go on it I said no. I was sat next to it behind the barrier and I just heard a big bang and I turn around and a little girl in pink flew up and back down.
"They did CPR on her straight away. She looked a lot younger than four-years-old.
"It is terrible. Cherish every moment you never know what could happen in a blink of an eye."
A joint investigation between the Health and Safety Executive (HSE), local authority and police has been launched into the circumstances around the incident.
The East of England Ambulance Service said it received more than a dozen calls about the incident.
A spokeswoman said: "We attended the incident this morning at Gorleston beach, involving a patient using inflatable equipment. More than a dozen calls were received at just after 11am, and several people at the scene including the Royal National Lifeboat Institution (RNLI) were providing assistance to the patient.
"Several of our teams were dispatched, with the first on scene in four minutes. The young female was seriously injured and in cardiac arrest on our arrival, and was conveyed to James Paget Hospital.
"Sadly, despite all the of the efforts and interventions, she was pronounced deceased.
"We would like to thank everyone who rushed to respond the young patient, and did everything possible to give her treatment and care. Our thoughts are with the family at this time."
The incident is the latest in which a fun day out with giant inflatables has turned to tragedy.
Married fairground workers William Thurston, 29, and Shelby Thurston, 26 - who were responsible for the "entirely preventable" death of a seven-year-old girl who was blown away in a bouncy castle they did not properly secure - were jailed for three years in June.
Summer Grant was killed after a gust of wind lifted the inflatable from its moorings and sent it "cartwheeling" 300m down a hill at an Easter fair in Harlow, Essex, a trial at Chelmsford Crown Court had heard.
The couple, of Whitecross Road, Wilburton, near Ely, Cambridgeshire, were both found guilty of manslaughter by gross negligence and of a health and safety offence following the incident on March 26 2016.
An inquest jury in May 2010 ruled the deaths of two County Durham women, who were killed when an inflatable artwork blew away, were accidental.
Elizabeth Anne Collings, 68, and Claire Furmedge, 38, died after the Dreamspace artwork blew free in Chester-le-Street in July 2006.
The huge walk-in structure - half the size of a football pitch - took off in a gust of wind with 20 visitors inside.
Ms Furmedge, from Chester-le-Street, and Ms Collings, from Seaham, died from injuries suffered when they fell from the artwork. Several others were badly hurt.
The inquest heard there were no detailed discussions about how the structure would be fixed to the ground.
