After four years of construction and nearly two decades of planning and debate, the last stretch of the Metro subway system's Orange Line is taking shape in the middle of Northern Virginia's rapidly developing I-66 corridor.

Four skylight-roofed stations have been built in the I-66 median. Nine miles of tracks have been laid. Electric power has been turned on, activating the 750-volt third rail on part of the new route. This month trains started running at night to test Metro's complex electronic control system.

The $220 million extension is expected to open in less than a year -- by late June or early July of 1986 -- and officials say the massive project is on time.

"You're in the 12-month countdown period," said Falls Church Mayor Carol W. DeLong, whose city overlooks two of the new stations. "The sooner, the better."

"We're planning on having it opened by July 4," predicted Fairfax Supervisor Joseph Alexander, a senior member of Metro's board of directors.

Transit planners and rail managers already have started preparing for the extension's opening. They are debating train schedules, analyzing ridership forecasts and calculating possible fares.

Under current rates, a trip from the Fairfax County town of Vienna, the Orange Line's prospective terminus, to the Metro Center station in downtown Washington would cost $2.30 at rush hour.

From the new West Falls Church station to Metro Center, the rush-hour fare would be $1.70.

At times other than rush hour, each ride would cost $1.10. These rates may be revised, officials said.

The transit authority is expected to consider raising subway and bus fares in mid-1986, and any change in rates would likely apply to the new stops.

Flanked by parking lots with 4,500 spaces, the four stations are expected to draw thousands of commuters from Fairfax and Prince William counties and from Falls Church, Fairfax City and Manassas.

Metro officials say the extension is likely to boost ridership by nearly 16,000 one-way trips a day.

On a recent morning, William S. Pauling, a construction engineer who has overseen much of the work on the extension, strolled through the West Falls Church station. Sunlight streamed through the station's overhead skylight and glass walls.

"I think this is a pretty neat station -- light and airy," he said.

Nearby, a subcontractor's crew was "browning the bronze" -- staining metal panels with a sulfur-based liquid to produce a uniform shade and luster. Along the tracks, communications workers laid cable.

Much work remains. Escalators, signs, Farecard machines, communications cables and other apparatus must be installed. The electronic equipment that regulates the rail system must undergo months of testing to ensure that the trains brake and accelerate properly in response to automatic signals.

The sprawling parking lots and access roads at the Vienna terminus have not been completed. Bus ramps -- the focus of a tangled conflict -- remain to be paved at the West Falls Church station. A special safety barrier is being erected along sections of the I-66 median to reduce risks of collisions between trains and cars or trucks.

Under a controversial 1977 decision by former secretary of transportation William T. Coleman Jr., part of the construction work was carried out by the Virginia Department of Highways and Transportation.

The ruling, which cleared the way for completing I-66 inside the Capital Beltway, required the state to furnish millions for mass transit.

Some officials have urged Metro to open the extension ahead of schedule. But when the isssue recently was raised by the Northern Virginia Transportation Commission, Metro officials said they still needed time to wind up the project. Moreover, they said, no funds have been set aside for an earlier start.

Any prospect of opening the East and West Falls Church stops ahead of the other stations was scotched several years ago by vehement protests from Falls Church.

The city objected to allowing West Falls Church, actually outside the city limits, to serve as a temporary terminus because of concern over a possible influx of traffic.

Under tentative plans, Metro would extend the Orange Line's current train schedules to the new stations, providing service every six minutes at rush hour and every 12 minutes at other times.

Other options may be considered, however. An alternative plan, officials said, is to offer slightly more frequent service and avoid the use of eight-car trains. All trains would then consist of six cars.

Among county and city officials, debate has focused on access roads and bus connections. Fairfax City, which runs a small bus system, has expessed concern about possible tie-ups on key roads near the Vienna station, such as Nutley Street where a $17.7 million state highway interchange is planned.

"The traffic is going to be horrendous. The buses won't be able to get through," warned Richard R. Fruehauf, the city's transit and utilites director. "We're afraid we're going to lose passengers."

Metro officials also say a planned extension of Prosperity Avenue to provide access to the Dunn Loring station may be delayed beyond opening day. In Falls Church, officials say they are seeking to deter commuters from using Haycock Road, a narrow street near a school.

Parking at the new Metro lots already is an issue. "I'll bet you that within a year, those parking lots will be completely full," said Fairfax Supervisor Alexander. "And people will be clamoring for more space."