The doors of ailing Chrysler Corp.'s Dodge Main assembly plant, one of the auto industry's oldest and largest factories, will swing shut Jan. 4, about six months sooner than originally planned, the automaker reported yesterday.

Production at the massive plant, which drew generations of eastern European immigrants after its construction in 1910 by brothers John and Horace Dodge and once employed 9,000 people, had been scheduled to end near the end of the 1980 model year.

In a statement, the No. 3 automaker said the production capacity of the Hamtramck, Mich., plant no longer is required, due to a severe decline in retail sales.

Fleetwood Enterprises Inc., the nation's leading producer of recreational vehicles and manufactured housing, has started closing four motor homes and three manufactured housing plants, a move that will result in nearly 500 layoffs, the company has announced.

One of the firm's plants in Riverside, Calif., that will be closed employs 39 people and manufactures van conversion, "a product which has not been significant in relation to our other RV lines and which the company will no longer produce," Fleetwood President William W. Weide said Thursday.

Fleetwood disclosed that the other three motor home plants to be closed are in Indiana, Michigan and Pennsylvania.