Washington area retail sales rose about 10 percent in the first four months of 1978 to $3.6 billion, according to the Commerce Department.

Department stores were relatively flat, however, and sales at the largest stores in downtown Washington continued to decline.

Overall sales at all types of retail firms including restaurants and car dealers, totaled $953 million in April compared with $978 million in March (a decline of 3 percent) and an estimated $893 million in April 1977.

The decline from March to April reflected the traditionally strong Easter selling season, which occurred in March this year.

Department store sales throughout the area were estimated at $111 million in April and $391 million for the first four months, about the same level as in 1977. Procedures for estimating retail sales have been revised and previous-year figures are not exactly comparable.

Sales at three major downtown D.C. department stores - not named by the government but thought to be Hecht's, Garfinckel's and Woodward & Lothrop - totaled $26.9 million in the first four months this year, down slightly from the 1977 estimates. However, the downtown sales of $7.65 million in April surpassed March volume by 10 percent.

In downtown Baltimore, sales were down sharply from last year. In April alone, sales of major department stores there fell 32 percent from the 1977 level to $2.9 million.

Total Maryland retail sales in April were estimated at $1.26 billion and, in Virginia, at $1.45 billion. Comparable figures for 1977 are not available.