CHICAGO, AUG. 14 -- . The heaviest downpour in 100 years today flooded hundreds of homes, blocked highways and left O'Hare International Airport, normally one of the world's busiest air terminals, an island almost inaccessible by land.

The record 9.35-inch rainfall brought flash floods to parts of the city and suburban areas, stopping commuter trains, stranding hundreds of motorists and knocking out electrical power to about 30,000 homes, officials said.

As much as 6 feet of water covered expressways, and some motorists had to be rescued from the roofs of their cars.

Four people died, authorities said. Richard Palluck, 35, was thrown through the windshield of his truck after it hit a flooded area on the Kennedy Expressway. Susan Bartlett, 23, apparently drowned when her car swerved into a ditch filled with eight feet of water on a state highway near Rochelle, about 70 miles west of here.

Jose Arellano, 26, was found in his flooded basement, where he apparently drowned after windows burst inward and water rushed in to ceiling level. Johnny McQuinston, 14, was electrocuted while on a delivery truck moving through a viaduct that had become electrically charged from flooding on a nearby railroad track, police said.

The storm hit northwest Chicago and the city's northern and western suburbs hardest. About 300 people fled their homes. One hospital had to evacuate patients, and high water forced two others to curtail services. State police said "the vast majority" of roads in northern Cook County were impassable this morning.

Gov. James R. Thompson (R) declared disaster areas of Cook and suburban DuPage counties, making residents eligible for state aid. Thompson activated 200 National Guard troops to provide security and help in evacuation efforts.

The rain began with rolls of loud thunder over Lake Michigan late last night, peaked with a driving downpour during morning rush hour and continued intermitently into early afternoon. Evening televison news shows proclaimed it "the flood of 1987."

The day's rainfall exceeded by more than 3 inches the previous record of 6.24 inches, set July 12-13, 1957. The National Weather Service said it was the heaviest rain here since record keeping began 100 years ago and predicted more for Saturday.

But the storm had an uneven impact. The South Side of the city and southern suburbs reported moderate rainfall and few problems. Life on downtown streets returned to almost normal by noon, and the Chicago Cubs played a full nine innings today at Wrigley Field on the North Side.

O'Hare, northwest of Chicago, was a different story. It became "virtually impossible" to reach when flooding closed the Kennedy Expressway from downtown to the airport, said Marilyn Katz of the city Department of Aviation. The Eden Expressway, which connects the city with its affluent northern suburbs, was closed in two places.

Only two of the airport's 12 runways were open late in the day, causing flight delays and cancellations, airline spokesmen said.