Total employment of District of Columbia residents declined by 4,800 in April, dropping the numbers of people with jobs below 300,000 for the first time in a year, the D.C. Department of Labor reported yesterday.

The seasonally adjusted unemployment rate for city residents climbed to 8.2 percent, an increase of 0.5 percentage points over March. But it still was below the 8.8 percent jobless rate in April 1978.

For the entire metropolitan Washington area, including the District, the seasonally adjusted unemployment rate in April was 4.5 percent, a decline of 0.2 percentage points from March and slightly below the 4.8 percent rate for the same month in 1978.

Nationally, 5.8 percent of the work force was listed as jobless in April.

In the District, the actual number of unemployed persons in April was 25,700, a decline of 100 from March. However, the rate of joblessness went up because the total work force - the number holding jobs and the number actively seeking jobs - dropped from 328,600 to 323,700.

The total number of Washingtonians who held jobs in April was 298,000, compared with 301,000 in April 1978.

In its analysis, the D.C. labor agency said the change during April was counter to the usual trend of increasing employment in the springtime. Officials said they could not identify why the trend had shifted.

For the metropolitan area as a whole, including both Washington and suburban residents, there were 1,449,100 persons working and 69,000 unemployed. The number of jobs increased by 19,000 over March and by 33,000 over April 1978.

The largest increases over the past year were 11,400 in service industries and 9,300 in wholesale and retail trade. There were 5,300 more government jobs, mostly with the federal government.