Thousands of federal workers may shift over to a 4-day week early next year. Some employes in computer and hospital operations could even volunteer for extra-long days of just over 13 hours to give themselves a 3-day week, and 4-day weekends.

The expanded workday, and the stretchout of some government work weeks to 6-day operations, will affect many others in and out of government. Firms doing business with Uncle Sam, the media, industries, contractors and special interest groups may find their hours changing, and workweek patterns shifting along with government.

People who now have to take time off from their jobs to get help with social security problems, medical services or income taxes should soon be able to visit key federal offices at nights or even on weekends, thanks to a combination of longer workdays and flexible shifts that permit Monday through Saturday hours.

Already more than 81,000 federal employes here are on flexible schedules. It allows them to set their own starting times, take longer lunches or mid-day breaks and permit their agencies to be open longer. Nationwide about 200,000 civil servants are taking part in the flexitime program that has changed the government from an 8-hour to a 12-hour operation. Employes still work 8 hours but may with supervisory approval, come in early or late, or take time off during the day so long as they work the regular number of hours.

Legislation recently signed by the president will encourage agencies to expand the flexible hours program. And, for the first time, it makes the 4-day (and even 3-day) week a very real prospect. Key to the change is a waiver that permits employes to elect to work 10-hour days if they will not put in for overtime after 8 hours.

The 4-day week has been held up in government by the law requiring overtime after 8 hours. Now that the longer days is possible at no extra cost, portions of Treasury and the Internal Revenue Service are already studying plans to put some workers on the 4-day week. Other agencies will experiment with it after April 1 when the flexitime test program expansion begins.

At least 21 agencies here already have some or all of their employes working flexible hours. The program has permitted longer service hours, changed commuter pattersn in some communities and, officials say, cutdown on tardines. Agencies already involved in flexitime include:

The National Institutes of Health in Bethesda; Army Computer Systems Command in Alexandria; Environmental Protection Agency; Geological Survey in Reson; Civil Service Commission; International Communications Agency; units of the Alcohol, Tocco and Firearms Bureau; Federal City College; Army Materiel Command; National Security Agency; Army Research Institute in Arlington; National Oceanic and Atmospheric Administration if Rockville; Library of Congress Copyright Division; David Taylor Research Center in Bathesda and Annapolis; Naval Air Station; Food and Drug Administration; Law Enforcement Assistance Administration; Civil Rights Commission and Agriculture's Economic Research Service.

Agency for International Development recently switched to a flexible hours schedule for many employes.

Goddard Space Flight Center in Greenbelt and the ACTION agency downtown are planning too.

The most intriguing possibilities from flexitime, the 4-day week, won't come on any major scale until April when new federal leave and holiday pay regulations - governing people on 10-hour days-are made official.

The Civil Service Commission which is leading the flexitime program for other agencies, plans a series of seminars here and around the country to acquaint employes and managers with the new regulations on flexitime. Those sessions should be held before Christmas in this area.