The blaze began shortly before 3 a.m. and was brought under control in about an hour, fire officials said. At least eight row homes were completely gutted, leaving behind only charred frames. The cause of the fire remains under investigation.
A fire department dispatcher identified the victims as 4-year-old twin girls, a 4-year-old boy and a month-old boy. The relationship between the twins and the two boys was unclear. Four other people were injured.
Jeff Boone told The Philadelphia Inquirer that he saw a couch on fire on the porch of a house about five doors down from his residence and heard children screaming.
The flames spread across porches so fast, he said, that "it looked like someone had a flamethrower and just shot it all across."
The 27-year-old Boone said he called 911 and bolted out of his house to try to save his neighbors.
"I was running, screaming, telling everybody, 'Get up! Yo, get yo! Go!'" he said.
Milton Musa told the newspaper his roommate woke him up and said their home was on fire. Once outside, Musa said, he saw two children hanging from a neighbor's window.
"I could see they weren't strong, and I was afraid they'd fall to the cement," Musa said. "So I went underneath them, let them fall on my back and carried them away."
He wiped away tears as he recalled the intense moments as the blaze swept through the neighborhood.
"Everyone was running for their lives," he said. "I've lost everything. My paperwork, my documents, my house. Everything."
A preliminary investigation indicated the blaze started in a couch at one home and spread quickly to other residences.
"This is a tragic, tragic day for the city of Philadelphia. Tragic," Fire Commissioner Derrick Sawyer said. "We lost four children today."
The Red Cross said 42 people were displaced by the fire.