The boys were so undernourished that they couldn't speak.
The four brothers, aged from two to six, were living in an apartment in Denver, Colorado, which was filled with faeces, cat urine and flies.
The boys were removed from the apartment late last month and placed in state custody, the police said.
Their parents, Wayne Sperling and Lorinda Bailey, were in court early this week on charges of felony child abuse.
Bailey, 35, is free on bail, while her husband, Sperling, 66, is in custody.
"These kids have lived in such a bizarre environment that they probably haven't developed any level of trust," said Ms Diane Baird, a licensed clinical social worker and paediatrics instructor at the University of Colorado School of Medicine.
Doctors found the boys were malnourished, not toilet-trained and had poor and delayed verbal skills, according to an arrest warrant affidavit. They made "infant-like noises" to each other, one officer reported.
The apartment had an unbearable odour like that of a decaying animal, AP reported.
After the boys were taken into protective care, neighbours complained that they'd reported abuse more than a year ago but no action was taken.