The children were reunited with their mother in November but continued to receive regular visits from caseworkers, said Carrie Proudfit, spokeswoman for the Florida Department of Children and Families.
It was a case worker – after the children's former foster mother went to their home and could not find them – who notified police, prompting the criminal investigation.
Three of the children were located yesterday and placed, again, in foster care, police reported.
The fourth, the twin of the infant who died, is the one missing.
Police investigators spent much of the day yesterday, interviewing Fryer. They arrested her early this morning.