Arlington County Board member Ellen M. Bozman announced yesterday she will seek a fourth four-year term this fall as an independent, a decision that is expected to bolster the diminishing political role of the nonpartisan coalition, Arlingtonians for a Better County.

Bozman, 59, is expected to face Republican Dick Herbst, a South Arlington civic activist and Commerce Department lawyer who is expected to announce his independent candidacy soon for the Nov. 5 election. Bozman's seat is the only one on the five-member board up for election this year.

If reelected, she will be only the second person since Arlington became a county in 1920 to win four terms on the County Board. F. Freeland Chew, who ran as both a Democrat and independent, served 16 years on the board from 1939 to 1955.

"I am a Democrat nationally and have been and will be," Bozman told a press conference yesterday. "But the background I came from says . . . it's very important to allow as many people as possible to participate in political campaigns."

A onetime federal employe who worked as a budget officer for the former Bureau of the Budget, Bozman's political roots are in the ABC, a self-described nonpartisan organization that traditionally has backed County Board candidates who run as independents with the Democratic Party's endorsement.

Once the dominant political force in Arlington elections, the ABC includes many federal employes barred by the Hatch Act from participating in party politics, but who have been a potent force in independents' campaigns. As more candidates have run as Democratic nominees in recent years, ABC has increasingly focused on providing informational forums on community issues.

"ABC has carved out a real role for itself in issue forums, a place where people can come and hear different views," Bozman said. A three-time County Board chairman, she is expected to win the endorsements of county Democrats and the ABC at their May mass meetings.

Currently the County Board is made up of Bozman, three Democrats and Michael E. Brunner, who ran as a Republican-backed independent in 1983 because he was a federal employe at the time.