Water use restrictions on more than 600,000 Northern Virginians were to be lifted last night, ending four months of a "crisis" that caused little serious hardship but did demonstrate the vulnerability of the Occoquan Reservoir.

Prince William County's supervisors voted yesterday to drop its curbs on water use as of midnight and the Alexandria City Council was expected to take identical action last night.

Fairfax County had set a midnight end to restrictions at its board's Monday meeting after officials from the three jurisdicti ons served by the Fairfax County Water Authority agreed that they should act simultaneously.

Fairfax City Manager George E. Hubler also announced yesterday that a weekend ban on outside water use affecting that city's 60,000 customers would be lifted at midnight Friday. It had been in effect since July 8.

The water authority had recommended last week that restrictions be lifted based on the beneficial impact on the reservoir of heavy rains in the last two weeks. Voluntary conservation measures had begun in early AUgust and mandatory restrictions on such things as outdoor watering, car washing, filling of swimming pools and use of water-cooled air-conditioners had been put into effect on Sept. 17 in Fairfax County and Alexandria and Oct. 1 in Prince William County.

The Occoquan Reservoir, the water authority's primary source of water, had dropped to a record low level of 1.85 billion gallons in late October before the rains sent it back to about 6 billion gallons.