The Fairfax County Board of Supervisors yesterday endorsed the concept of a 4-percent gasoline sales tax in Northern Virginia, but added a qualification that some legislators called divisive.

The board, in a special meeting, voted 8-to-0 to give a qualified endorsement to a regional gasoline tax proposal now pending in the state legistature, but added it wants a say in what happens to money that the tax would raise.

That puts the county at odds with some legislators who say that the taxes should be funneled directly to the Northern Virginia Transportation Commission. It could apportion the money on the basis of the Metro costs of the various suburban jurisdictions served by the transit system, under the pending legislation.

The supervisors insisted yesterday the county should keep revenues it generates. The proviso would eliminate the possibility that Arlington and Alexandria, which bear a higher proportion of Metro's Virginia costs, could get some of the gasoline tax revenues generated in Fairfax.

State Del. Warren G. Stambaugh (D-Arlington) said the Fairfax action could make his efforts to get a Metro tax bill through the House of Delegates "much harder. They come down here and half of them say one thing and half say another . . . We're trying to get them the money they need and they end up nitpicking us to death."

Another complication that surfaced at yesterday's meeting in Fairfax is the question how much revenue would be raised by a 4 percent gasoline tax in the Virginia suburbs. The Northern Virginia Transportation Commission staff said revenues for the fiscal year beginning July 1 amount to $37.2 million, or slightly more than the $35.7 million Metro deficit estimated for that same period.

But Fairfax County's transportation staff said revenues would amount to only $22.09 million, which would mean the localities will have to produce more than $13 million from other revenue sources to pay for Metro costs. The gap would continue to grow each year, the staff said.