A small survey of parents in Philadelphia found that three-quarters of their children had been given tablets, smartphones or iPods of their own by age 4 and had used the devices without supervision, researchers reported on Monday.

The survey was not nationally representative and relied on self-reported data from parents. But experts say the surprising result adds to growing evidence that the use of electronic devices has become deeply woven into the experience of childhood.

Dr. Michael Rich, the director of the Center on Media and Child Health at Boston Children’s Hospital, said he suspected that exposure to mobile devices among children elsewhere “is not all that different” from what was described by the parents in Philadelphia.

“Based on my observations of families with whom I work, I would not be surprised if these levels of device ownership and use were similar in many families,” he said.