The regional task force that is studying the future of Metro unanimously approved for further assessment four alternative rail systems to the existing 100-mile Metro plan in a watershed meeting yesterday.

After months of difficult discussions, thousands of pages of consultants' reports, several angry exchanges with the federal government and a significant number of internal regional disputes, Francis Francois' committee of area politicians and technicians nailed down the final outline of the federally mandated study.

There is still a month or more of work to be done before a final report containing four alternatives to the present system can be forwarded to local and federal governments for additional comment.

Nonetheless, it is clear that the Metro system will be changed to a greater or lesser degree from the original outline that was traced on the regional map almost a decade ago.

Sixty miles of the Metro system already have been approved and funded and will not be changed. Four more miles has federal and local blessing but lacks final funding. Still another 4.5 miles - the controversial extension of the Red Line from Silver Spring to Glenmont - has local blessing, no funding and persistent federal opposition.

But the regional task force assumed, after consultation with the federal government, that all those segments would be completed. The question then became one of what would happen to a total of 30 miles on four suburban legs: the Greenbelt Line from Gallery Place through Northeast Washington; the Branch Aveneu line from Anacostia; the Vienna Line from Ballston in Western Arlington County, and the Franconia line from King Street in downstown Alexandria.

The four alternative systems adopted all assume that the Branch Avenue line from Anacostia will be completed to Branch Avenue and the Beltway, and that it will be rerouted from its present alignment to include a stop at St. Elizabeths Hospital.

There are two other assumptions on the Virginia side of the Potomac River: the proposed line from Alexandria to Franconia, if built at all, will not include a stop at Springfield and the line from Ballston will be completed as far as the Beltway either to Vienna as now planned or to Tysons Corner.

With those assumptions in place, here are the four alternative systems that will receive further study:

No line from Alexandria to Franconia: a Vienna terminal on the line from Ballston: the Columbia Heights station as the end of the line from Gallery Place, with no extension into the Prince George's County.

No line from Alexandria to Franconia; a Tysons Corner terminal on the line from Ballston; a Columbia Heights Terminal on the line from Gallery Place.

The Van Dorn Street station would be the end of the line from Alexandria: a Tysons Corner terminal on the line from Ballston; a complete Greenbelt line beyond Columbia Heights.

All the Franconia line would be completed; a Tysons Corner terminal on the line from Ballston, a complete Greenbelt line.

Those alternatives range from what committee members described as a minimum system - no Franconia line and terminals at Vienna and Columbia Heights - to a maximum system that would have most of the Metro completed as presently planned, with slight changes.

The prime consultant for the study - Peat Marwick Mitchell and Co. - will now generate ridership estimates for the various combinations, along with estimates on what it will cost to build and operate the various systems.

The regional task force - which contains representatives from the Metropolitan Washington Council of Governments, the regional Transporatation Planning Board, and the Metro board - also adopted yesterday criteria that will be used in judging one system against the other. Obviously, transit ridership, projected automobile usage and energy consumption will be considered, along with costs of building and operating the rail systems.

Land use and other local planning considerations also will enter into the final report and public hearings are planned in some jurisdictions before the final paper is forwarded to the federal government.

While the consultant is preparing new numbers, technical committee will be working on an accompanying report to satisfy several additional federal questions. The future of the Glenmont line remains the major unresolved area, at least in the minds of the federal governments.

Local governments must also develop by the end of August a financial plan on how they will pay for building and operating the Metro system ome preliminary work has been done. "Perhaps we'll proceed in parallel with five financial plans," Metro board chairman Joseph Whole said yesterday.

The restudy of the system was ordered in November 1976 by the Department of Transportation. Metro's original construction cost of about $2.5 billion - first projected in 1969 - has more than doubled and the money has run out. The study is to determine whether the rest of the system is justified.

Francois, a Prince George's County Council member who himself called for a Metro reassessment at the beginning of his one-year term as chairman of the COG board in 1976, is credited with a masterly job of keeping the study moving.

"We now have a mass transit menu," he told a reporter yesterday. "It's a question of how much the diner can afford."