An afternoon snowfall iced roadways and seriously snarled the evening rush hour yesterday, causing four traffic fatalities and numerous other accidents and delays as commuters headed home for the start of the last weekend before Christmas.

Light snow, no more than an inch as of last night, began falling in the region about 12:30 p.m., according to the National Weather Service. By early evening, it had tapered off to flurries. Forecasters said to expect occasional snow flurries today on the first day of winter, which officially begins at 5:08 p.m.

The snowfall trapped thousands of motorists in traffic, and even President Reagan got a taste of a premature rush hour: With his helicopter grounded by the weather, the president made the 70-mile journey to Camp David, Md., by motorcade, a trip that took two hours.

In Montgomery County, a Silver Spring woman was killed about 2:30 p.m. when her car was struck by a vehicle sliding on a snow-slickened road.

Police said Satish Gupta, 41, of 12605 Montclair Dr., suffered head injuries and was pronounced dead at Washington Adventist Hospital. Police said a car driven by Thomas Harris, 53, of 3 Castaway Dr., Rockville, was northbound on New Hampshire Avenue when Harris lost control of the vehicle and it crossed the median strip and struck Gupta's car. Harris was charged with negligent driving and failing to keep to the right of the center of the road, police said.

Two other accidents, one on Montrose Road in Montgomery County and the other involving a school bus on the Suitland Parkway in Prince George's County, claimed the lives of an Alexandria man and a woman who remained unidentified by police last night.

According to Montgomery police, Charles Robert Keyes, 44, of 519 N. Armistead St., died during surgery at Suburban Hospital after suffering massive injuries when the Volkswagen Beetle he was driving slid into the path of a car driven by Barry Stuart Marcus, 35, of 6 Orchard Dr., Gaithersburg. Four others were treated for injuries in that accident.

U.S. Park Police said a woman died of injuries suffered shortly before 3 p.m. when her car, traveling west on the Suitland Parkway, crossed the median and was struck by a Prince George's County school bus. Neither the bus driver nor the three students on the bus were injured, police said, but the unidentified woman was taken to Malcolm Grow Air Force Hospital at Andrews Air Force Base, where she died.

And in the fourth fatal accident just shortly after 2 p.m., U.S. Park Police said Minerva Etzioni of Bethesda was driving southbound on the George Washington Parkway near the Memorial Bridge when she lost control of the car and struck a tree. Etzioni was taken by ambulance to National Orthopedic Hospital where she died later.

The weather, coupled with it being Friday and the Christmas season, was an inducement for most commuters to try to get an early start on the weekend. Many federal and District government employes left their offices early, hoping to get out of the city while the snow was just dusting the roadways.

But in a city where even dustings of snow sometimes cause the same traffic delays as major storms, few made it.

"The traffic's awfully bad, worse than rush hour," said Park Service spokeswoman Sandra Alley.

District traffic engineer George Schoene said the Department of Public Works called in its full fleet of snow removal trucks and de-icing equipment after the lunch hour.

At 1:30 p.m., he said, traffic signals for outbound commuters had been switched to rush-hour cycles, almost three hours earlier than usual. And by 3:30 p.m., more than 45 accidents had been reported in the District, according to traffic reports.

Traffic attempting to leave the city during the afternoon and early evening crawled or remained stalled for hours. Similar problems occurred in the suburbs. Metro officials said all of the 1,600 buses it uses during rush hour were 30 minutes to an hour behind schedule on every route.

Fairfax County school buses were running up to 90 minutes late in the afternoon. Spokeswoman Dolores Bohen said the buses were involved in 13 fender-bender accidents, with none inflicting any injuries.

The cold and snowy weather was expected to hit the homeless hard. Mitch Snyder, the Community for Creative Non-Violence spokesman, said he was expecting about 700 homeless people to stay at the group's shelter last night.

Juanita Jones, clothing director of Bread For The City, sat in her tiny office near Thomas Circle and watched people in need pick through piles of donated clothing.

"Everyone is in here looking for coats, gloves and hats today," she said. "But we don't have much to give them. In the winter we get summer clothes donated and in the summer we get all the heavy stuff."

On K Street NW, pedestrians, often laden with brightly wrapped holiday gifts, gingerly negotiated slippery sidewalks and threaded their way around and through the stalled and slow moving cars.

All along the K Street office-building canyon, automobile horns honked in a long cacophony of frustration that was punctuated from time to time by the sirens of emergency vehicles.

On the Mall, however, only a few yards from slow moving streams of traffic, there were moments of calm beauty. From the Washington Monument, Park Ranger Michael Moreno admired the unsullied white swath that covered the broad green lawns and the Reflecting Pool.

"It's lovely til it gets spoiled," he said of the broad blanket of snow.