The D.C. Board of Education approved last night a proposal that could triple the number of city 4-year-olds who receive prekindergarten instruction.

The expanded program is set to begin this fall, but pupils will be accepted only on a first-come, first-served basis because it is unclear how much money will be available for it in the budget for the coming year.

The fiscal 1984 school board budget is now before Congress.

Currently, about 3,000 4-year-olds are enrolled in half-day and full-day prekindergarten programs, but the board's action could open instruction to as many as 6,000 more. James T. Guines, associate superintendent for instruction, estimated the total cost of the expanded program at $3.5 million to $5 million.

Approval of the expansion proposal came on a motion from Bettie Benjamin (Ward 5), who noted that 15 percent of the city's elementary schools offer no prekindergarten instruction. R. Calvin Lockridge (Ward 8) expressed concern, however, about financing the program. "It does not make sense to make a policy and then operate in the dark on funds," Lockridge said.

In other action, Chief D.C. Superior Court Judge Carl Moultrie I swore in new Ward 1 board member Edna Frazier-Cromwell, who was recently appointed to fill the seat vacated by City Council member Frank Smith Jr. Moultrie also swore in the new student board member, Howard Miller II.