Patriotic Front guerrillas and Rhodesian police, long bitter enemies, carried out four joint patrols in the last week, British military officials said today, providing the first hesitant sign that the country's warring armies may merge peacefully.

In all but one case, Commonwealth cease-fire monitoring forces joined in the patrol. The missions have been limited to those areas where the guerrillas assembled under the terms of last month's Rhodesian peace settlement signed in London.

Although four limited patrols are a far cry from a melding of the hostile military forces, British officials regard them as a significant move.

It is if Arab and Israeli troops carried our joint patrols with participation of a U.N. peacekeeping force a month after the cease-fire in the Middle East wars of 1967 or 1973.

Tenuous as the development may be, such removal of the barriers raised by seven years of bloody warfare could help forestall the possibility of renewed fighting after next month's election of a black-majority government.

In fact, the British hope that joint patrols will be carried out before the election on Feb. 27 to 29 in all 14 areas where almost 22,000 Patriotic Front guerrillas have assembled.

Both coleaders of the Patriotic Front forces, Robert Mugabe and Joshua Nkomo, have called repeatedly for integration of the forces.

The subject is touchy because the Rhodesian military generally has regarded the guerrillas with disdain and has been reticent to talk about unification of the forces. It is noteworthy that so far the patrols involved the Rhodesian police but not the Army.

A spokesman for the British governor, Lord Soames, said today that the future shape of the Army is for the elected government to decide, but he added that plans for retaining some of the guerrillas for civilian jobs is being discussed by British, Rhodesian and Patriotic Front officials.

At first, both sides registered surprise when approached by the monitoring forces about the patrols, British military officials said. They have agreed to have joint patrols in all areas, but details have yet to be worked out.

In all cases, one official said, the patrols are under the command of the police, since they are responsible under the peace agreement for law and order during the transition period. He said no problems had been encountered in the patrols and the guerrillas had carried out orders from the police.

Another official said he believed it was "possible to meld the armies once they have broken down the intransigence on both sides." He added that the barriers were "crumbling."

He also said authorities hoped that by the time the election the cooperation will be good enough to allow Rhodesian local officials to administer the camps rather than having the monitoring forces do it.

Success in this area could help ease a potentially volatile situation facing the British at the end of the election when tentative plans call for the monitors to move out of the camps.

Many analysts fear that move could leave the way open for renewed warfare between the Rhodesians and the Front. If the 550 lightly armed monitors remain at the camps, however, they could be in a precarious position if some parties are upset by the election outcome.

The most significant of the four patrols occurred Monday about 50 miles west of Bulawayo near the Botswanan border. Four policemen and four guerrillas joined together, without the assitance of the monitoring force, to disarm and return a group of Patriotic Front supporters to their assembly camp.

The maneuver involved retrieving the guerrillas from a village two miles from their assembly point known as Lima. Under the peace agreement, the guerrillas can lawfully stay in the country only if they remain in their camps.

The other three patrols involved two or three members of the Commonwealth force, armed with submachine guns. The police are equipped with either their standard rifle or a pistol while the Patriotic Front forces are armed with their traditional Soviet-designed AK47 rifles.

Two of the patrols occurred at Lima and a third at Lupane, about 100 miles northwest of Bulawayo. Both are in areas where guerrillas loyal to Nkomo are assembled. The other one was carried out with Mugabe's force at a Marymount mission across the country in the northeast section near the Mozambican border. Two occurred this week and two last week. Each lasted about two hours, the officials said.

In another military development today, the British colonial government announced that South African units had withdrawn from the Rhodesian side of the frontier at the Beitbridge border crossing point and were replaced by Rhodesian forces.

The withdrawal came just hours before the U.N. Security Council was to begin a debate on alleged British violations of the London agreement, including the presence of South African troops in Beitbridge.

The move is expected to defuse the debate somewhat and deflect attention from other complaints such as deployment of Rhodesian forces and auxiliaries loyal to former prime minister Abel Muzorewa to help contain guerrillas who have not assembled.

Soames originally allowed an estimated 200 South Africans to guard the bridge but after numerous international complaints South Africa agreed Saturday to remove the troops once they could be replaced by Rhodesians.

Mugabe and Nkomo maintain that there are thousands more South Africans still in the country in violation of the London agreement, but Britain says any such forces are under Rhodesian military command and therefore authorized to be here.