The Fairfax County Police Department's 702 patrol officers will switch next month to a four-day workweek of 10-hour shifts, the department announced this week.

The new "four-ten" format, a one-year pilot program approved unanimously this week by the county Board of Supervisors, will "increase the patrol availability of officers during the peak hours of police activity, without an overall increase in the number of patrolmen," said Police Chief Richard A. King.

"Under the current system," King said, "we realized that a limited number of officers were available between the hours of 8 p.m. and 2 a.m., the time during which police work is in its highest demand. In effect, the 10-hour shifts will create an additional overlap of police shifts during the time when we're needed most."

Currently, patrol officers work seven straight days with the following two days off. One officer, Cpl. Don Keeley, said a problem with that system was that officers found it "difficult to really be alert for seven straight days."

King said the new plan, which begins Feb. 23, would "meet the needs for a change of patrol officers" and would allow officers more flexibility for part-time jobs. For instance, one patrolman said, depending on the shift, officers will have one to nearly four days off under the new system. Police spokesman noted that about 275 officers hold part-time jobs to supplement their regular income.

Police in Arlington, Prince George's and Montgomery counties already have adopted the four-ten shift. King said Fairfax considered the plan in 1978, but a shortage of police cruisers prevented its implementation. "The Board (of Supervisors) approved 33 new cruisers for the (present) fiscal year, which gave us the opportunity to start now," King said.

He added that a countywide survey of patrol officers this fall indicated that 85 percent favored the new working hours. "That's a significant figure," King said, "and it required that sort of support before we could implement (the plan)."

Officers in the Groveton District in southeastern Fairfax County have been working the new shift on an experimental basis since July, and most officers seem to favor the plan.

"It makes a big difference," said Groveton officer Keeley. "You're more alert. Plus, I commute to Fairfax from Manassas and go to school two nights a week. It really helps you cut down on traveling expenses."

Police spokesmen said about one-third of the officers live outside the county.

Senior patrol officer Doug Graves, however, said the new shifts might not help officers who have outside, part-time jobs.

"Under the new shift," he said, "we're not allowed to work part-time on working days, and the days off vary with each week. Police work is the primary concern, but that might hurt someone's availability for part-time work."

King said standardized hours for the new shift would be discussed in the coming weeks.