Four weeks from today, hundreds of thousands of people -- maybe more than a million -- are expected to be crowding on the Mall. Some will have spent several nights there, sleeping out in the brisk October air. Others will be arriving in chartered trains and hundreds of crowded buses.

Thousands more may walk from satellite parking sites to the Mall in what one official called a vast "pilgrimage." All will be there to see one man.

Pope John Paul II.

It will be the last day of his week-long, six-city tour of the United States: the first time that a pope has ever toured this country, the first time one has ever visited Washington, the only time in their lives that many American Roman Catholics will ever have a chance to see the leader of their church.

With perhaps the biggest assembly in Washington's history -- the papal mass on the Mall scheduled for 3 p.m. Oct. 7 -- less than a month away, church, metropolitan and federal officials are working frantically to sort out their plans.

What will the pope's motorcade routes be? How many streets will be closed? Who will be invited to the reception at the White House when the pope meets with the president Oct. 6? What exactly will the stage and altar for the enormous mass look like -- and will they be finished in time? How many medics are needed? How many souvenirs to sell. How many portable toilets? No one is certain yet.

Nothing quite like this has ever happened in Washington before.

One of the most delicate tasks the planners will have to tackle is the decision on just who will get how close to the pontiff.

The Rev. Ronald Jameson, in charge of planning the liturgy for the Mall service, said recently that a select group of 150 people are expected to receive communion directly from the pope. If possible, the archdiocese is hoping to avoid a mad scramble among area Catholics striving to be among the prechosen 150.

Jameson, normally an irrepressibly cheerful and enthusiastic man, said he couldn't sleep nights when he first learned he would be involved with winnowing out the the chosen few. More than a few feathers will be ruffled by the final decision.

The selection system has yet to be decided, but Cardinal William W. Baum and Archbishop Jean Jadot, the apostolic delegate in Washington, will make the final determinations.

Another agonizing choice will be the three area families who bring the communion hosts to the pope during the mass. Officials have already decided there must be one white, one black and one Hispanic family to reflect the composition of the Washington archdiocese, a church spokesman said.

The basic schedule for the pope's tour of the United States and his itinerary in Washington are largely fixed, in some cases down to the minute. But requests for special consideration from individuals, groups and some of America's most influential leaders continue to pour in to church offices.

The pontiff is scheduled to arrive in Boston Oct. 1, address the United Nations in New York Oct. 2, visit Philadelphia Oct. 3 and Des Moines, for a few hours, Oct. 4. He will spend the nights of the 4th and 5th in Chicago, then arrive in Washington at about 11 a. m., Oct. 6.

But despite the apparent finality of this schedule, Cleveland and Baltimore officials are still pushing for the pope to visit their cities, according to church sources. Sen. Howard M. Metzenbaum (D-Ohio) plans to call the Vatican tomorrow to press Cleveland's case, his press aide said.

Eighty senators signed a letter to House Speaker Thomas P. (Tip) O'Neill last week asking him to invite Pope John Paul to address a joint session of Congress while he is in Washington. Church officials said the pontiff probably will not have the time.

After arriving at Andrews Air Force Base on the morning of Oct. 6, plans call for Pope John Paul to be flown by helicopter to a site by the Lincoln Memorial reflecting pool, then proceed in a motorcade to St. Matthew's Cathedral near the corner of Rhode Island and Connecticut avenues NW.

There he will probably celebrate a mass for area priests, eat lunch at the rectory and possibly address the crowds from the rectory balcony.

The pope will arrive at the White House at 1:30 p.m. for a private meeting with President Carter and then the Carter family. Afterwards, a reception is planned in the East Room and on the South Lawn for more than 4,000 guests, including members of Congress, the judiciary and other branches of government. The public should be able to watch from the Ellipse.

At 4:15, Pope John Paul is scheduled to go to the Organization of American States for a short meeting with Latin American diplomats before arriving at the apostolic delegate's residence on Massachusetts Avenue across from the Naval Observatory at 5:30. There will be a reception for the diplomatic corps and an 8:30 dinner at the delegation before the pope retires there for the night.

Sunday morning Oct. 7, as the multitudes begin thronging onto the Mall, the pope's itinerary has him in Northeast Washington at the Shrine of the Immaculate Conception. He is expected to address about 7,000 nuns there, then go to the gymnasium at nearby Catholic University to talk to about 2,500 members of the academic community. By 10:30 a.m., he will be meeting and praying with several hundred leaders of other Christian denominations. He will then return to the apostolic delegation for lunch and rest.

By then the Mall will probably be packed. Streets will be closed for several blocks around. Metrorail and buses will be bringing people in, although Metro spokesman Cody Pfanstien said last week how many are in operation will depend on whether union employes can be persuaded to work on the Columbus Day holiday weekend.

Current plans call for a chain link fence to surround a 10,000-square-foot, three-tiered stage in front of the Smithsonian Castle. Local artisans will have worked night and day to construct the platform and altar dominated by the papal chair 13 feet above the ground, according to its architect, Robert J. MacMahon.

Within the fenced area -- "the sanctuary" -- will be from 3,000 to 5,000 seats, church officials said. They will be reserved for visiting cardinals and bishops, a 1,500-member special choir, and an ambiguously defined group called "guests of the Holy See."

A small number of handicapped may also be allowed in the sanctuary.

According to Deacon Chris Ruggles of the papal information center that has been set up at St. Bernadette's church in Silver Spring, these guests will be selected by Baum and Jadot. No tickets will be sold.

Ruggles said he has already received numerous calls from "congressional type people" who want to get a reserved seat close to the pope. Another planner involved with the visits said he has been plagued with inquiries from "people who consider themselves VIP's who may not be" for the same purpose.

Outside the fence, more than 4,000 ushers recruited from local Catholic parishes are expected to help the fully mobilized police forces of the Washington area manage the crowds. No tents or trailers will be allowed on the Mall, but many people are expected to sleep there under the stars the night before. The Smithsonian museums and restaurants will be open until 9 p.m. Saturday and Sunday, Oct. 6 and 7.

The pope and his retinue will probably leave the apostolic delegation for the Mall shortly after 2 p. m. He is expected to put on his religious vestments near the Mall. The entrance procession along Jefferson Drive should begin at 2:55.

The mass will take more than two hours, with communion for the multitudes gathered outside the fence distributed by about 1,500 priests and deacons despersed through the crowd. Though women have acted as eucharistic ministers in many American parishes, they will not perform that task for the papal service.

While planners are spending most of their time working on the complex logistical problems for all this, they will have not forgotten such little pleasantries as papal memorabilia.

The Rev. Desmond Murphy, chairman of the "procurement committee," has ordered 40,000 papal flags, 1,500 commemorative coins and medallions, 10,000 glossy pictures of the pope and 100,000 pocket-sized holy cards at a cost of $37,000.

Murphy said these items will be sold through Catholic schools and parishes before the pope arrives, but he is concerned that street vendors will "disrupt the dignity" of the Mall mass by hawking Pope John Paul tie pins, mugs, buttons and T-shirts. Murphy said a shirt with the pope's likeness and motto that says "Take a Peek at the Pope" is already circulating around town.

At 5:30 p.m., Oct. 7 the pontiff will leave the Mall and at about 7 p.m. he will depart Andrews Air Force base and the United States for the trip back to Rome.

About 500 boy and girl scouts are being recruited to help clean up the Mall on Columbus Day.