Stepdad grilled in death of B’klyn kid, 4

A 4-year-old boy was found dead in Brooklyn on Sunday morning, and his stepfather was taken into police custody for questioning, cops said.

Neighbors informed cops that the dead child’s 6-year-old brother told them he saw their stepdad at some point assaulting the younger boy, an NYPD spokesman said.

The child’s mother was holding the tot when she waved down police in front of her home on Baltic Street in Gowanus around 5 a.m., police said.

Officers attempted CPR on the child, who was rushed to Brooklyn Hospital. He was pronounced dead a short time later, said police.

Around the same time, someone called 911 stating that a man was attempting to kill himself at the address, authorities said. Police who responded found that it was the stepfather, and he was taken into custody for questioning, authorities said.

The man was later taken to Bellevue Hospital for a psych evaluation.

“The investigation will determine how this child died and what the stepfather’s [possible] role was in it,” a police spokesman said.

The boy’s death came two days after a 23-year-old Bronx man was arrested in the brutal, fatal abuse of his girlfriend’s 1-year-old son.

In that case, Keishawn Gordon is accused of breaking tiny Legacy Beauford’s ribs and sodomizing him with a toothbrush to stop him from crying.

Gordon later told cops that the baby was “irking me’’ with his cries and claimed he wasn’t throwing the child in the air “that hard” to get him to stop, according to court papers.