For the fourth consecutive year, the House national parks subcommittee approved expansion of the Manassas National Battlefield Park yesterday, but unlike before, Virginia's two U.S. senators now appear ready to support some enlargement of the park.

The perennial proposal to add 1.715 acres to the 3,000-acre park, site of two Civil War battles, will be discussed at a meeting today on Capitol Hill between freshman Sen. John W. Warner (R-Va.) and the Board of Supervisors of Prince William County, where the park is located.

A majority of the seven-member governing board has opposed the expansion plan, saying it would remove valuable commercial property from the fast-growing county's tax rolls, to expand a park seldom used by local residents.

Each year since 1976, the House has passed a version of the expansion bill offered by Rep. Herbert E. Harris (D-Va.) only to have it blocked or killed in the Senate by either Sen. Harry F. Byrd Jr. (I-Va.) or Warner's predecessor, Republican William L. Scott.

An aide to Warner said today's meeting is aimed at reaching a compromise with the Board of Supervisors, so that Warner can introduce his own, modified plan to expand the park. A spokesman for Byrd said the state's senior senator stands ready to support "an alternative proposal that has the support of area residents."

The Prince William board contends that up to 80 percent of the county's residents oppose enlarging the park. But the expansion is supported by a wide array of historical and preservation groups.

Anne Snyder, chairman of the Save the Battlefield Committee, told Rep. Phillip Burton (D-Calif.), chairman of the House subcommittee, yesterday that unless the park is expanded, "the only place where the South won two battles . . . will become a nightmare of gas stations and hamburger joints."

"I'm just glad that the Grand Canyon isn't in Prince William County," Snyder said, "because our Board of Supervisors would want to use it as a sanitary landfill."

Last year, the Prince William supervisors suggested a compromise plan limiting the areas that could be included in the expansion. They quickly dropped their compromise offer when it appeared no one else would support it.

Yesterday, Board Chairman Kathleen K. Seefeldt read a statement reiterating the board's opposition to the park's expansion. Seefeldt had sided with the minority when the board voted 4 to 3 last February to oppose the House bill.

Supervisor Donald L. White, an opponent of the bill, accused proponents yesterday of "deliberately misleading" people into believing that unless the park expands Manassas will turn into "another Gettysburg, where poor planning has turned that area into a fast-food, garish-looking strip of commercial development." The board "has never advocated such development and I don't believe it ever will," White said.

One refinement in Harris' 1979 proposal, however, addresses one of the supervisors' concerns involving "in-holders," persons who own property within the present boundaries of the park. A new provision would guarantee in writing that their homes and land could never be acquired without their consent by the National Park Service, which operates the park.

Historical properties added to the park by the bill include the Brawner farm and Gibbon woods, where the Second Battle of Manassas (Bull Run) began in 1862; the Stone Bridge area, where a year earlier Union troops made diversionary attacks that began the first land battle of the Civil War. Wheeler farm, headquarters of Confederate Gen. Joseph Johnston during the first battle and an area south of Northern Virginia Community College, east of Rte. 234 and north of interstate Rte. 66, where Confederate troops staged their counterattack on the Union flank in both battles, also would be added.

Sources said the latter area, the main entrance to the park, is the area the county is most interested in for commercial development. It also is the area most likely to be involved in any compromise offered by Warner.

Only 1,200 of the 1,715 acres to be added to the park under Harris' bill are in Prince William County. The rest is in Fairfax County, where county supervisors have not opposed the expansion.

Harris' bill now goes to the full House Interior Committee.