The incident happened in front of 10 Meserole Street around 12:18 p.m.
The NYPD released surveillance images of the suspect on Monday.
One relative pleaded for the boy's mother and her boyfriend to turn themselves in to police.
The city's medical examiner will determine the boy's cause of death.
The 20-year-old is being held in jail without bail on murder and criminal possession of a weapon charges.
The 4-year-old boy's identity has not been released as of Thursday afternoon.
The victim was transported to Brookdale University Hospital, where he was pronounced dead.
The 4-year-old was transported to Maimonides Hospital in critical condition, where he was later pronounced dead, police said.
Winston Nguyen, 38, has been charged with enlisting a child in a sexual performance and five counts of endangering the welfare of a child.
The incident happened on a Manhattan-bound L train near the Graham Avenue subway station in Williamsburg Friday afternoon.
The 32-year-old driver has been charged with manslaughter in the second degree and criminally negligent homicide, among other charges.
Police have been called to the location 29 times so far this year.
The officer was charged with DWI, DWAI, leaving the scene of a collision with injuries and refusal to take a breathalyzer test.
The victim has not been identified, pending family notification.
In January, Brian Dowling pleaded guilty to second-degree murder.
The victim was transported to Brookdale Hospital Medical Center, where he was pronounced dead.
The blaze started around 8:30 a.m. at 1237 67th Street in Dyker Heights.
The blaze started around 10:30 a.m. at a home on Eastern Parkway between Brooklyn and New York avenues
A 37-year-old victim was taken to Kings County Hospital, where he was pronounced dead.
The incident happened at a Macy's store located at 422 Fulton St. in Downtown Brooklyn.
The incident happened in front of 55 Gerry Street around 4:50 p.m. on Saturday, police said.
More than 60 firefighters and 12 FDNY units responded to the scene, officials said.
Vehicles blocking fire hydrants slowed the response, an FDNY fire commissioner said.
A 15-year-old boy was arrested and charged on Thursday, police said.
The victim was taken to NYC Health and Hospitals/Kings County, where he succumbed to his injuries.
The victim suffered a gunshot wound to the leg, police said.
The suspect shot a 25-year-old man twice in his shoulders.
The victim suffered a gunshot wound to the leg, police said.
The incident happened near 415 Lefferts Avenue around 6 p.m., police said.
Officers found a 15-year-old boy in the lobby of the building with a gunshot wound to the head.
The incident happened inside E. 92nd St. in East Flatbush on Friday, police said.
BREAKING: The fire broke out on Graham Avenue, officials say.
The victim — identified as Shaliek Smith —was approached by a group of men and was stabbed in the chest.
Two men allegedly attacked Michael Shelonchik, 53, in the back of a livery cab.
Michael Shelonchik was found unresponsive in the back of the vehicle, police said.
The suspect has been charged with attempted murder, assault, reckless endangerment, menacing, and criminal possession of a weapon.
The second incident happened at a deli in Brownsville, police said.
An adult and two children were transported to a local hospital in critical condition.
The teens all face robbery, assault and harassment charges, police said.
One victim has yet to be identified, and the shooter remains at large, police said.