The Ugandan Foreign Ministry announced today the execution of four white men believed to be West German and Swedish journalists caught entering the country to cover the war after crossing Lake Victoria by canoe.

The two West German and two Swedish journalists were last seen Thursday in the Kenyan lake port of Kisumu trying to rent a craft after being turned back at the Kenyan-Ugandan land border.

Although the Ugandan Foreign Ministry identified the persons executed as mercenaries, Western diplomats said the victims almost certainly were the journalists.

Feared dead were Swedes Arne Lemberg, 38, of Expressen newspaper and Carl Bergman, 31, of Svenska Dagbladet, and West Germans Wolfgang Steins, 30, Nairobi correspondent of Stern magazine, and Hans Bollinger, 34, a photographer for the Paris-based Gamma agency on assignmen for Stern.

Bollinger was held in a jail in the Ugandan capital of Kampala for several days two weeks ago after flying in without permission.

West German diplomatic sources said the four were picked up soon after landing not far from the Kenyan border in a coffee smuggler's canoe, taken to a nearby police station and shot shortly thereafter.

If the journalists' identities are confirmed, the deaths will be the first killings of Western journalists in Uganda since American freelancer Nicholas Stroh, scion of a Detroit brewing family, was executed by Ugandan troops in 1971 while investigating reports of a massacre in army ranks.

Swedish Embassy sources here said they were still checking out vague reports that another group of four whites had been arrested by Ugandan authorities and that three of them were said to be still alive.

Meanwhile, President Idi Amin's affort to stave off defeat appeared to be paying off as his recently arrived praetorian guard fought back against Tanzanian troops and invading Ugandan exiles five miles south of Kampala.

Committing the West Nile Battalion made up of Nubian mercenaries and his own Kakwa tribesmen yesterday apparently signaled the embattled Amin's determination to make his last stand in Kampala.

David Martin, a British journalist who was the first to expose Amin's excesses, wrote in his book "General Amin": His praetorian guard are his Nubian mercenaries and trusted Kakwa. They have killed so much that they must continue killing for fear of the revenge which will inevitable be unleashed once Amin falls."

The new tough line emerged with Commerce Minister Mohammed Bakhit warning over the Radio Uganda that unless businessmen and residents returned to Kampala and opened their shops and offices by Wednesday they risked having the premises reallocalted to long lists of would-be owners.

An appeal yesterday for Kampala residents to go back to work today went largely unheeded, according to diplomatic sources.

Symptomatic of the perhaps only momentary change of fortunes for Amin was the sudden depression and circumspection shown by anti-Amin Ugandans both inside and outside that country.

Ugandan residents of Kampla whoe in recent weeks have chatted openly with telephone callers and from Nairobi and elsewhere now have gone back into their shells. They have taken once again to saying they prefer not to talk about matters of substance.

Residents who dared with speak reported seeing many more troops-and even armored personnel carriers-in Kampala streets. Some exile sources placed their numbers as high as 3,000. Even if exagerated, that would represent a considerable accomplishment for an army which only 10 days ago was reported to be throwing away its arms and uniforms and fleeing.

Actual fighting appeared to be focused mainly around a coffee plantation at Lubowa, about five miles south of Kampala, according to informed sources.

Nigeria, black Africa's most populous and richest state, issued an appeal for restraint and indirectly castigated Tanzania in condemming outside interference in Uganda.

Expressing publicly the fears elsewhere aired only in private, Radio Lagos warned that if the principle of outside noninterference in other states' affairs were violated for whatever reason, then few militarily powerful nations could determine leadership in other countries and set off an Africa-wide chain reaction.

Nigeria's army is by far the largest military establishment in black Africa.

United Press International added from Nairobi on the plight of foreigners in the war zone:

According to diplomatic sources, two European businessmen, Briton Gordon Parrott of Atlas Tower Construction Co. and Dane Sven Sorensen of Bitumastic Ltd. in Uganda, were picked up by Tanzanian troops near Kampala and were reported safe.

Another newsman, Jan Stage from Copenhagen, Denmark, said in an interview with UPI that he escaped Uganda after being interrogated and beaten in Kampala by Amin's secret police.

Stage said he was pummeled with rifles, kicked by troops, forced to crawl around a room and robbed of most of his possessions and money before being rescued by diplomats, escorted to the Kenyan border and released. CAPTION: MAP, Missing journalists' reported route. By Richard Furno-The Washington Post