A New York manufacturer whose defective dehydration remedy is believed to have caused the deaths of four Peruvian infants has been sentenced to prison, fined and ordered to repay $266,000 after pleading guilty to defrauding the U.S. agency that purchased the preparation.

Mohammad Haleem Kahn, owner and director of U.S. Materials Co., was charged with misrepresenting his company's ability to fulfill a $266,000 contract for 2.8 million packets of oral rehydration salts for use in Peru.

Some packets produced by Kahn's firm contained excessive potassium, resulting in the babies' deaths in a Lima hospital in March 1986, a spokesman for the U.S. Agency for International Development (AID) said yesterday.

Kahn, 46, of Spring Valley, N.Y., was sentenced to three years plus two years probation, fined $1,000 and ordered to perform 100 hours of community service by a federal district court judge in New York yesterday.

Kahn pleaded guilty to defrauding AID. Federal prosecutors agreed to drop 31 other charges, including four counts of involuntary manslaughter.

A sentencing memo handed up during Kahn's trial said he had employed teen-agers as young as 13 to prepare the hydration mix in "filthy conditions" at his Garnerville, N.Y., factory, according to prosecutor Franklin Stone of New York.

Rehydration salts are widely administered to infants suffering from diarrhea and malnutrition and have been credited with reducing infant mortality, the AID spokesman said.

Some 200 million packets of the salts are produced annually. Jay F. Morris, acting AID administer, said this was the first instance of a problem of this kind in the six years the agency has been distributing the product.

"Stricter controls and procurement procedures, such as inspection of contractors by the Food and Drug Administration, have been adopted to prevent such mishaps in the future," an AID statement said.

An AID spokesman said that no formal approach for compensation had been made to the agency. Earlier reports indicated that the government of Peru planned to seek $20 million.