It's a big wedding at the Four Seasons. But big: four separate food tables, each serving a different cuisine; a ballroom packed with people; a seven-piece band that is just knocking itself out. Everyone is feeling great. A rock 'n' roll medley has left the dancers redfaced and excited, jackets flung away, shirttails pulling loose. The pace picks up. The wild strains of the Hora start them all clapping, bouncing to the beat. The bandleader shouts into the mike and now the bride is being hoisted hilariously aloft in a chair . . .

"We run the party," said Steven Niederman, a singer and bass player who is a co-owner of Sixpence. "We know what makes a party work. We've all been in top show bands, but we're not just musicians. We're entertainers. That makes the difference. We play to entertain the people, not to hear what we might want to hear."

The four partners who form the core of Sixpence have been together nine years, which in the weekend band business is forever. There are four other regulars. All the men are around 30, and most have daytime careers, but the music is what they live for.

"I'm a CPA," said partner Mark Grant, who plays piano and organ, "and it's a busy time. But here I am."

They play maybe eight gigs a month: weddings, debutante balls, Christmas parties, bar mitzvahs, industrial balls, political affairs. They handled the Bullets' opening game at the Capital Centre. They played at the White House for Ford's inauguration.

Huge weddings are such an old story to them that they often stage-manage the rehearsal, especially if they are to play at the ceremony.

"We practically live at the Four Seasons and the Mayflower," muttered Grant. "We paid our dues, though."

What makes a party roll, said Tim Theoharis, singer and guitarist, is continuity. If the buffet suddenly opens in another room, guests drain away and the excitement deflates. "Sometimes the caterer wants the music to stop because the roast is done; more often the roast isn't done and so we play through our break and make the caterer look good."

You get a sharp sense of timing after you've logged a few hundred wedding receptions. You know when to give the young crowd a rest and play waltzes, and when to try a tarantella (at Italian weddings) or a jig (at Irish), and when to start the Hora.

"It used to be traditional at the wedding of the last daughter of a Jewish family," said Niederman, "but everyone does it now, at bar mitzvahs, even at Italian weddings."

Sometimes Cousin Branwell ("such a lovely voice, you know") wants to join the band and they let him unless the host has ordered otherwise beforehand (which happens more than you'd think). The musicians know how to cover for him so skillfully he thinks he's jamming with them. Now and then they run into an amateur who's very good indeed. Accomplished in many forms of music from rock to show-biz stuff to the big-band sound, they can pick up a new song in a few minutes, often come to a job 15 minutes early to smooth out a number they have rehearsed separately. Their own style, when they indulge it, runs to Manhattan Transfer jazzy-swing melodies with lots of vocals. The fourth partner, by the way, is flutist-singer George Taylor.

Sixpence is so popular now--almost entirely by word of mouth as they get passed along from one wedding host to another--that they are booked (via the Washington Talent Agency, which handles about 14 bands plus variety groups) well into 1983 with calls for certain dates as late as 1985. Three hours on a Saturday night with seven musicians and two 20-minute breaks will run you about $950. It goes up if you want a cocktail pianist or a wedding ensemble as well.

"It's a big high for this area," said Grant, "but a good band can make a great party."