Open for lunch Monday through Saturday 11:30 a.m. to 2:30 p.m., for dinner 6 p.m. to 10 p.m. daily, Sunday brunch 11 a.m. to 3 p.m. AE, CB, DC, MC, V. Reservations suggested. Prices: At lunch, appetizers from $2.75 to $12, main courses from $10.75 to $13.75; at dinner appetizers from $3 to $35, average $7, main courses $12 to $22, average $18. Full dinner with wine, tax and tip about $50 per person.

The mansion with the "mostest" is back in business. The figures are said to have run into the millions to groom the graceful old Fraser mansion for its reentry in the haute restaurant circuit. And it shows. After the valet has taken your car from the circular driveway you enter an expanse vast enough for--and furnished to resemble --a grand hotel lobby. If you aren't stopping first in the beautiful lounge downstairs, you ascend one of Washington's most grand staircases, and the ma.itre d'h.otel leaves his antique desk to show you to a table in one of four rooms. The treat in the rear room is the massive stained- glass bay window; on the left is a room of more intimate scale with flowered silk wallpaper. In another room you might find your favorite of the 16 fireplaces in marble, stone and carved wood, or a detail in the carved plasterwork or the ceiling. Some tables are too close, and some chairs are less comfortable than others, but there are variations to fit your mood among the 125 seats, and upstairs are sumptuous private banquet rooms.

The restoration of grandeur is also in the Royal Doulton china, the tall white candles in silver holders and the flowers in tiny bowls that catch your eye but do not obstruct your view. And beginning dinner with a champagne cocktail allows you to enjoy the tall, graceful fluted glasses.

After a hesitant start, Fourways seems to have settled on a polished cadre of waiters, wearing double-breasted white jackets and dealing knowledgeably with food and diners. One waiter describes a long list of specials, then volunteers that he will be happy to repeat any or to tell you their prices-- a neat way of dealing with the verbal-menu problem. Waiters are also willing to help you choose among the 18 main dishes and even more appetizers, most of which sound irresistible. Quail eggs and caviar in creamy chive sauce have not been seen elsewhere, but then there is also a crab custard appetizer or a green pea soup. Main dishes run to seafood mousses, either as stuffing Dover sole or standing alone; truffles and morels decking beef or veal; port wine cream with lobster; or vintage port with tournedos. They sound so poetic that Fourways' computer-printed checks, with each dish named in full and presented in duplicate, have become souvenirs.

But does the food taste as good as it sounds? More often than not, yes. That soft and fragile crab custard nearly melts on your tongue; in fact, the appetizers have been outstanding. That green pea soup was far from humble, being fresh, creamy and full-flavored with a few peas and tiny cubes of what tasted like chicken quenelles, though the waiter had said it was garnished with chicken livers. The,e of salmon and lobster was a beautiful platter carefully paved with slim vegetables; the,e itself was held by the barest binder of seafood mousse wrapped in thin slices of carrot and accompanied by a creamy watercress sauce and a punchy tomato aspic. It cost an outlandish $10.50 but showed what a seafood,e ought to be. A few weeks later, scallop,e was less enticing; though light and airy, it was slightly gelatinous and its layer of green was too grassy tasting for the delicate seafood. Warm oysters in beurre blanc were faultless. And at one Sunday brunch, the cold poached salmon was velvety, fresh-tasting and prettily decorated with scallion and pimiento flowers under a thin aspic. The only real disappointments among appetizers have been meat,es; a sampler at brunch were handsome, but there was little flavor to remember. And the quail eggs were beautifully prepared--slightly runny yolks, excellent caviar-- but the chive cream was so rich it overwhelmed the eggs.

Two lessons might be learned from the appetizers: Fourways prepares seafoods beautifully and does better with the extraordinary than the ordinary. For main dishes, veal has been well chosen, well trimmed and cooked with care; but one day its morel cream sauce was merely good, nothing memorable; another day the escalope was chewy, honored by fresh truffles but topped with an ordinary brown sauce. Homemade noodles with tomatoes and basil butter, too, had no special character yet didn't call for complaining. Duck was so soft it might have been steamed before roasting; its bite was missed, though it was agreeably fat-free. Its sherry-vinegar and grape sauce--the evening's special--was light and clear, with impressive touches such as peeled grapes and a side dish of pommes souffl,es. But, in all, the duck preparation was merely good. And brunch's chicken liver risotto was chicken livers cooked in a forgettable brown sauce to a thorough grayness and spooned onto plain rice.

Seafood main courses have been another matter. Lobster was taken from its shell and set in a slightly sweet port cream, a lovely interplay of restrained sweetness. Dover sole was a whole fish wrapped around a coral-colored, silky lobster mousse. Salmon as a main course was as soft and moist and fresh as it had been at brunch, and set in a delicious beige cream. Unfortunately, it was on a bed of acrid sorrel that should have been diluted.

Along with main dishes come cunning little vegetables: tiny batons of glazed carrots and turnips, buttery mixed juliennes and warm peeled cherry tomatoes. You could order chanterelles, fried eggplant and pommes souffl,ees (mistranslated originally as "apple souffl,e" on the menu), but they are not necessary. Salads are pretty, but not inspired. And desserts, though beautiful, do not thrill the palate, whether the slightly grainy and insufficiently flavored souffl,e, the elaborate torte that tastes of nothing special or the cr,eme caramel that is too firm and bland. Spectacular-looking, yes. Respectable-tasting, sure. Knockouts, no. If you insist on dessert, try a fruit tart with a crisp and buttery crust, a wisp of custard and some specially exotic out-of-season fruits.

The coffee at Fourways not only is very good, it is poured from a handsome pot. But such a restaurant ought to offer brewed decaffeinated coffee rather than instant.

Fourways will have a fine wine cellar in a few years. Even now it has a few grand old wines, and some good values from the '66 and '67 vintages. But the list reflects the restaurant's newness and while the choice of French and California wines is long and varied, I can hardly imagine anybody who could afford a $225 Roman,ee-Conti being willing to drink a '79 now. Besides nearly all the wines being '79s or younger, their prices are high. The choice of wines under $20 will offer something complementary to the food but not ideal.

Fourways is grand, from its conception to the details of its execution. It aims for the top. And with a little settling and aging, it has a good chance of getting there.