Four murals by Jack Beal, the realist painter, were unveiled here last night at the Department of Labor. They are nice reflections of their theme, the History of Labor in America. They took a lot of work.

They're heartfelt and they're big (each is 12 feet square). Their many costumed actors (the Indian, the trapper, the scientist, the hardhat, the capitalist in striped pants, the union maid, etc.) strike dramatic poses in dramatic settings (a seaside wood at dawn, an outdoor blacksmith's forge, a 19th-century mill, a 20th-century lab.) The lighting is theatrical. Beal's compositions, with their swooping curves and bunched diagonals, are as complicated as his interwoven plots.

Each of Beal's four pictures deals with a century, with energy, with what we've done to the environment and with a time of day. "Colonization," the first, is set in the 17th century at dawn, a campfire is burning, nature is unsullied. Note for future reference the stick upon the ground. In "Technology," the fourth mural, the century is the 20th, light comes from flourescent tubes, it's night, and the nature is in trouble. The scientist in the glass now-rare stick.

Beal's pictures are full of incident, props and art historical references - to baroque chairoscuro, to 19th-century landscapes, to the classical Laocoon. He built a studio in which to paint them. The commission was $150,000.

"We wanted something ordinary working people could relate to," says Dick Conn of the Labor Department. That's what they got.

Labor Secretary Ray Marshall; Robert T. Griffin, acting administrator of the General Services Administration; Michael Straight, deputy chairman of the National Endowment for the Arts; artist Beal, and Joan Mondale, wife of the Vice President, spoke at last night's unveiling. The murals are installed in the lobby of the Labor Department Building at 3d Street and Constitution Avenue NW.