For more than six weeks, a loose-knit group of young men has been terrorizing residents of southwestern sections of Prince George's County, accosting them as they arrived home from work or shopping, sticking a gun into their faces and taking their money and automated teller machine cards.

The business of the men, four of whom were arrested early yesterday, was robbery, police said. For a short time they conducted it at a maddening pace, robbing as many as three victims in an hour and a half some evenings, striking a dozen times over a weekend.

The men used the same getaway cars over and over, returned to the same neighborhoods and scarcely varied their routine at all. Almost always the men demanded -- and often received -- their victim's secret bank card code and went straight to the bank to cash in on it.

"Right from the beginning we knew their patterns," said Sgt. Burl Thompson, with the investigative division in Oxon Hill. "We knew we would catch them sooner or later. And we did."

Shortly after midnight yesterday, the men struck again, taking the checkbook and bank card from a man in the 5900 block of Fisher Road in Temple Hills, and heading for a nearby Sovran Bank.

This time, however, Pfc. Bernard Joseph spotted a van allegedly used by the robbers. Police arrested three men after they arrived at the bank branch in the 6300 block of Livingston Road. After questioning them, police arrested a fourth suspect a short time later.

Police declined to release the names of the men, saying that several more arrests are likely. The group, police believe, is responsible for at least 30 armed robberies since the first of the year in the Oxon Hill area alone. Investigators are looking at similar cases elsewhere in the county and in the District, Thompson said.

"We've had other robberies out here, but this group is the group that was causing us the most trouble," said Maj. John Moss, commander of the Oxon Hill District. "These people were doing it every night."

Police assigned two detectives, Cpl. Steve DelGiudice and Cpl. Allen Dischinger, and as the number of incidents mounted, brought in numerous officers to work in plain clothes, an unusual move that, sources said, underscored the seriousness of the problem.

A large percentage of the robberies in Prince George's County and the District traditionally have occurred in the late night and early hour mornings in areas known for drug trafficking. These cases, however, were different in that the robbers generally worked from about 7 to 10 p.m. and targeted middle-class and working-class people returning home from work or shopping.

Typically, the victims were women. Most often, the victims said they were threatened with what they described as a shotgun or a rifle.

In one case, which police described, two of the men approached a 39-year-old woman who was returning to her apartment on Keating Street in Temple Hills about 9 p.m.Tuesday after a trip to the nearby Safeway store.

The men grabbed the woman by the hair and pushed her back into her car, jamming what she described as a shotgun into her face, police said.

The men escaped with the woman's purse and bank card.

None of the victims in those cases was seriously injured. The robbers, police said, treated their work like a "three-hour-a-day, part-time job."

Thompson said that police believe the men bought expensive clothes and other items with their stolen money.

Several new purchases, complete with dated purchase receipts, have been found.

Armed robbery in Prince George's County increased by 23 percent from 1989 to 1990. While complete statistics are not available, police said that the number of robberies has continued to rise throughout the county.