DALLAS — The deaths of four children in hot cars in recent days has brought the number across the United States this year to at least 23, nearly matching the total for all of last year and prompting experts to plead for vigilance and warn parents that it can happen to anyone.

‘‘It just breaks your heart,’’ said Janette Fennell, founder and president of KidsAndCars.org, a national child safety nonprofit based in Philadelphia. ‘‘We’ve done so much to try to get the word out and maybe that’s why last year was down a bit but this year is not looking very good.’’

Fennell, whose organization tracks such deaths, said the number began dramatically rising in the 1990s with the passing of laws requiring that young children be placed in the back seat to avoid air-bag injuries.

The numbers of heatstroke deaths of kids in cars fluctuated in the following decades, averaging 37 such deaths a year since 1998. Last year, with about two dozen deaths, was an unusually low year. The worst was 2010, with 49, according to both a count by Fennell and Jan Null, a research meteorologist at San Jose State University, who also tracks numbers.