The deaths of the four men brought the dangers of living on the street into full public view, renewing attention on the unsheltered homeless, who make up only about 5 percent of the estimated 79,000 homeless people in New York City.

And the Bowery, an area of Lower Manhattan known in the 1930s as New York’s skid row, has long served as a daunting example of the city’s difficulties in addressing street homelessness. If anything, the problem is getting worse: The area is now drawing younger homeless people, many with drug and mental health problems, a population that the city’s traditional outreach methods have struggled to confront.

The homelessness crisis has been the most intractable problem of Mayor Bill de Blasio’s tenure. His $1 billion initiative to address mental health, ThriveNYC, has been criticized for failing to meet the needs of severely mentally ill people. The administration has also been slow to open new shelters as planned, leaving an estimated 3,600 people — many of them men who are emotionally disturbed and struggling with addiction — on the street.

Mr. Santos is accused of wielding a metal bar and swinging it at the men’s heads as they were asleep on the ground on a night that saw temperatures dip to 49 degrees. One of the men who was killed, Cheun Kok, was 83 years old, the police said. A fifth victim, a 49-year-old man, remained in critical condition on Sunday, officials said.

When spotted by a police officer at 2 a.m. on Saturday, Mr. Santos was carrying a metal bar covered in fresh blood and hair, according to a criminal complaint.