More than 4,000 civilian and military positions at the Pentagon will be eliminated under a sweeping economy move ordered by Defense Secretary Harold Brown.

The cuts are scheduled to be made between now and Feb. 1, 1978, in response to Brown's demand for leaner headquarters staffs in his own office, the Army, Navy and Air Force.

Although most of the reductions will be accomplished through transfers to the field, defense officials conceded yesterday that some people will have to be fired to achieve the requested staff cuts of between 20 and 25 per cent.

Brown started off the drive on Monday by announcing that his staff at the Pentagon would be reduced by 448 people from 2,065 to 1, 617, and that field staffs linked to his office would be cut by 344, from 1,539 to 1,195 civilian and military personnel.

He directed his deputies at the Pentagon to follow his lead by imposing cuts of between 20 and 25 per cent in the staffs that work for the civilian service secretaries and the uniformed ciefs of staff.

The Army, in response to Brown's direction, assembled a team to recommend what positions could be eliminated. The team's paper is scheduled to be in the hands of Army leaders no later than next Friday.

The other services are making similar efforts.

Although defense officials did not know precisely when the cuts would be imposed, they said yesterday that the notifications would probably start going out next month to employees to be transferred, retired or fired.

Brown has instructed Pentagon executives to achieve a balance between civilian and military positions in making their reductions.

Jobs that are vulnerable to elimination for the Army, for example, are the staff positions under Army Secretary Clifford Alexander and Gen. Bernard W. Rogers, Army chief of staff.

The cuts of between 20 and 25 per cent will come from the following numbers of civilian and military positions, defense officials said last night in giving what they said was a rough count:(TABLE) (COLUMN)Civilian(COLUMN)Military Army(COLUMN)3,210(COLUMN)1,941 Navy(COLUMN)1,899(COLUMN)1,397 Air Force(COLUMN)2,300(COLUMN)4,000 Total(COLUMN)7,409(COLUMN)7,338(END TABLE)

That total of 14,747 positions, if reduced by Brown's top goal of 25 per cent, would mean 3,687 fewer staff jobs at the Pentagon. Counting the 792 positions Brown has said he will eliminate from his staff, the total reduction could be as high as 4,479 positions.