A four-alarm fire believed to have been caused by an arsonist broke out yesterday at the Magruder's grocery store in Vienna, destroying most of the supermarket's inventory, Fairfax County authorities said.

The fire began shortly before 11 a.m. in a front room of the store at 180 Maple Ave. West in the Vienna Shopping Center, according to Fairfax County Fire Department spokeswoman Pam Weiger.

About 100 customers and employes were evacuated from the store, Weiger said. No injuries were reported.

The fire, which was reported at 10:53 a.m., was extinguished in about a half-hour, Weiger said, adding that about 100 county fire and rescue personnel responded to the alarms. Police cordoned off the block surrounding the store.

Weiger said flames shot through the roof of the market and spread throughout much of the store. An adjoining store, the Vienna Rexall Drug Center, received smoke damage, Weiger said.

Weiger said no dollar amount could be placed on the damage. A Magruder's executive at the scene who declined to give his name said the store contained extra inventory because of the upcoming holiday. He called the week before Easter "one of the two or three busiest weeks of the year."

The executive said he could not predict when the store would reopen. "We have to have the roof fixed," he said.

Office manager Susan Gore said that an employe called her attention to "flames on top of the walk-in freezer case" in the storeroom and that she immediately evacuated the store, which had opened at 9 a.m. Gore said employes had worked throughout the night stocking the store's shelves.

In May 1984 a fire that apparently was caused by an electrical short circuit broke out at a Magruder's store on upper Connecticut Avenue in the District, causing an estimated $150,000 in damage.

Magruder's Grocers, founded in 1875, has 16 supermarkets in the metropolitan area, Vice President Mark Polsky said yesterday.