If you are one of the area's 300,000 white-collar feds whose salary just went up 4 percent, enjoy the new pay raise while you can. It won't be with you for long.

Come January, paychecks for some of the poorest feds will shrink like an all-cotton shirt washed in hot water.

Higher health premiums and the new 1.3 percent Medicare tax on the way are going to do some strange and not-so-wonderful things to the take-home (the part that counts) pay of many civil servants.

Some feds have done the math already.

A Grade 14 with the Federal Communications Commission says the $31.40 biweekly raise he just got will be offset next year by a $34.75 bite on his check for his higher health insurance tab and the new Medicare tax that is being put on all government workers.

A GS 5 clerical employe with Health and Human Services says next year she will take home about 65 cents a week less than she is getting now.

The House Post Office-Civil Service Committee has just completed a study of the 4 percent pay raise and the offsetting effects of the 24 percent average hike in health insurance premiums and the new Medicare tax, which will cost feds anywhere from $100 to about $340 a year, depending on salary.

The committee, dominated by Democrats, argued for a "minimum" 5 percent federal pay raise this year. The White House, dominated by Republicans, set the raise at 4 percent.

The "average" employe cited in the committee workup is assumed to be a Maryland resident (for tax purposes), to have three deductions and to be enrolled in the popular Blue Cross-Blue Shield high-option family plan. Maybe you can find yourself here:

The starting GS 3 clerk just got a $410 raise. But that employe's health premium under this example will increase $247 next year; retirement contributions will be up $29; federal taxes up $64; state (Md.) taxes up $17 and the new Medicare tax will be $104. Net loss to the employe in 1983: $51.

A Grade 5 secretary got a $515 raise. But after higher health premiums, taxes and retirement payments, and Medicare tax, the employe will take a net loss of $14 next year.

Grade 7 starting pay is up $637. After all the new deductions, the employe will realize an actual take-home pay raise next year of only $17.

The increase for a GS 9 worker (with a $779) gross pay raise, is a take-home gain of $26.

Grade 11 workers will get a $942 pay raise, which will work out to a net gain in 1983 of $51.

A Grade 12 employe, in this model, will take home $62 more next year than he or she got this year. The raise for GS 13 is $63.

Senior executives, ($58,500) who got no pay raise this year, will take home $592 less next year. All of which proves it is better to get a raise than not to get a raise, but not much better.