The Fairfax County School Board last night approved a fourth elementary school closing for next year because of declining enrollment.

The board voted unanimously to close Pine Ridge Elementary in Annandale at the end of this school year, shifting its students to neighboring Mantua and Camelot schools.

The Pine Ridge closing--the most controversial of the four approved--came after weeks of study and hours of emotional hearings.

The three other elementary school closings--Pimmit Hills near Falls Church, Lewinsville in McLean and Virginia Hills in Annandale--were accomplished with the acquiescence of parents who requested the move in an effort to avoid emotional confrontations with the board in the future.

But that was not the case last night as parents of children who attend the three Annandale area schools could not agree which should be closed. Board members spent more than an hour explaining to almost 100 parents that there were not enough students in the Pine Ridge, Mantua and Camelot schools to keep all three open.

Board member Robert E. Frye accused his colleagues of being afraid to tackle tough decisions on school closings. "This board has been much too timid about reviewing closings of schools," he said. "We're going to have to take a very close look at more closings."

A study showed that at least 26 of the county's 120 elementary schools could become targets of consolidation studies because of declining enrollment.

While the school system is losing students in the eastern part of the county, it is planning to build three new elementary schools in the western part where population is increasing.