Police in Rockford say the attacks occurred following a home invasion on Wednesday afternoon. One of the injured is in a critical condition.
The victims are a 15-year-old girl, a 63-year-old woman, a 49-year-old man and a 22-year-old man.
A man, 22, has been arrested and is being questioned by police.
Police said the motive for the attacks remained unclear.
The home invasion occurred on Florence Street but the attacks unfolded at multiple locations, police say.
Winnebago Country Sheriff Gary Caruana said a young woman had sustained stab wounds to her hands and face after being seen running away from the suspect.
The woman is now in hospital and is being intubated.
Another man, described by Sheriff Caruana as a “good Samaritan”, sustained less serious stab wounds after trying to intervene to help the woman.
Rockford Police Chief Carla Redd said that not all of the victims had been stabbed, but added that none had suffered gunshot wounds.
She said police did not believe there were any other suspects at large.
“Words can’t even express my thoughts right now…” she said. “My heart goes out to the families right now who are suffering a loss.”
Rockford resident Ruth Gallagher told CBS her long-time postman was among the dead, while the US Postal Service confirmed that a postman had been killed.
“He was the kindest person,” Ms Gallagher said. “He would help anybody, would step in and help. Always friendly. It’s just devastation for all of us. It’s just a nightmare.”
In a statement, Rockford city mayor Tom McNamara said his heart was “breaking for those who have lost their lives, their families and our community”.
“This is, obviously, a huge blow to all of us,” he said. “However, in Rockford fashion, we get knocked down, sometimes too much, but we always get back up stronger.”
He added that free counselling services would be made available on Thursday and Friday at Flinn Middle School in Rockford.
