Hechinger Co., a chain of 47 retail stores specializing in home-improvement products, yesterday reported record first-quarter earnings of $3.8 million, up 12 percent from $3.4 million in the same period last year.

Earnings per share for the quarter were 21 cents compared with 19 cents last year. The latest profits marked Hechinger's 41st consecutive quarterly earnings increase.

Hechinger's sales for the quarter were up 30 percent to $114.6 million from $87.9 million in the first quarter of 1984.

In a related development yesterday, Hechinger announced a five-for-four stock split in the form of a stock dividend on both its class A and class B common shares.

It is Hechinger's eighth stock dividend and split in seven years. The stock dividend will be paid June 17 to shareholders of record on June 7. The company said that the annual dividend on its class A shares, currently 16 cents a share, will remain at the same level after the split, effectively providing a 25 percent increase in the class A dividend. The dividend on class B shares, which has been 10 cents a share, will be reduced to 8 cents a share on the larger number of shares.

* Potomac Electric and Power Co. reported net income of $41.4 million (77 cents a share) for the first four months of 1985, a slight increase over $41.2 million (76 cents) earned in the same period for 1984.

Operating revenue for the first four months was $378 million, up from $340.9 million in the year-ago period.

Pepco's net income for the 12 months ended April 30 was $168.3 million ($3.24), up from $153.4 million ($2.93).