Allentown Fire Capt. Lee Laubach says the fire started shortly before 4 a.m. Monday.
Firefighters had to force their way into a second-floor apartment to rescue two children and two adults. Heavy smoke disoriented the family and forced a 5-year-old girl to be admitted to a hospital.
She was listed in critical but stable condition.
Six other people inside the home sustained minor injuries.
Investigators say the fire appears to have been deliberately set on the back porch.
Incendiary material found there has been sent to the state crime lab.
Neighbors say the family returned on Monday night to retrieve what was left of their belongings.
"There's nothing left. Probably stuff in the garage. There's nothing in the house, everything is destroyed," said Maria Burgos.
Investigators are reaching out to the public for any information about this arson case.