Four months after the introduction of martial law, Poland's military rulers today announced they would release 1,000 internees and abolish the night-time curfew beginning Sunday.

An announcement issued after a meeting of the ruling Military Council for National Salvation said that 800 internees would be freed and another 200 allowed to return home "on vacation." This would leave between 3,000 and 4,000 activists of the independent Solidarity union movement still interned, plus several hundred more serving prison sentences for violating martial law.

At the same time, Poles learned officially that a planned visit here in August by Pope John Paul II almost certainly will be postponed. The news was contained in a report, broadcast on radio and television, on a meeting in the Vatican between the Polish-born pontiff and the Roman Catholic primate of Poland, Archbishop Jozef Glemp.

The radio reporter quoted Glemp as saying that the current political situation in Poland made it difficult for the pope to come here at present but that the visit might be rescheduled for next year.

A Polish prelate accompanying Glemp told The Associated Press in Rome that the primate is "concerned because the pope has set definite conditions which unfortunately are not present." He did not elaborate. Vatican sources were quoted as saying the pope would not want to give any impression that he accepted the restrictions of military rule by visiting Poland while they were in force.

The pope, who was last here in June 1979, had been invited to take part in celebrations marking the 600th anniversary of the arrival in Poland of the nation's most revered religious symbol, the Black Madonna of Czestechowa.

The reported postponement of his trip came as little surprise. Church leaders feared that another visit by the pope might be taken as a de facto endorsement of martial law while the Communist authorities saw it as a possible political challenge. The pope's 1979 visit to Poland is widely viewed as having served as a catalyst leading to the establishment of Solidarity in 1980.

In contrast, today's relaxation in the martial-law decrees appeared designed to show that, four months after the military takeover, the government is in full control. It also seems clearly intended to help persuade Western governments to lift economic sanctions including a credit freeze that has resulted in major supply problems for Polish industry.

Among the internees who have been released already is Jan Kulaj, the head of the rural branch of Solidarity. The 24-year-old Kulaj was shown on television tonight meeting Deputy Premier Roman Malinowski, the head of the Communist-affiliated United Peasants' Party.

In an interview also shown on television, Kulaj gave broad support to the government's agricultural policies. Since Rural Solidarity had been suspended, he said he would work with the Peasants' Party for "the good of my country and for farmers."

Kulaj's statement was undoubtedly a propaganda coup for the authorities as he is the best-known Solidarity leader to agree to appear on television. While he refrained from criticizing the union or recanting his political beliefs, the very fact of his appearance will be regarded by many Solidarity supporters as collaboration.

Kulaj became head of Rural Solidarity after leading farmers in several occupations of government buildings in 1980 and a lengthy court battle for the union's recognition. Known for his fiery speeches and love of fine suits, he was detained shortly after the declaration of martial law Dec. 13.

The Ministry of Interior, in today's announcement, said that visa regulations for foreigners wishing to visit Poland, including businessmen and tourists, would be eased beginning next month and working conditions for foreign correspondents improved. Automatic telephone communications within Poland would be restored on May 10.

A television announcer warned viewers, however, that martial law was still in force and that people breaking martial-law regulations, including a ban on strikes, were still liable to internment or arrest under summary procedures.

The announcer called on Poles to take part in May Day celebrations next Saturday organized by the Communist Party. Stringent security precautions are being taken for the occasion, which will mark the first time since the military takeover that the government has attempted to bring its supporters into the streets.

Leaflets from underground Solidarity activists urging a mass boycott of the May Day parades have been circulating in Warsaw. They suggest instead that people should either take a walk in the park or attend masses in churches.

Solidarity has also announced plans for a follow-up to the eight-minute broadcast from a clandestine transmitter that was heard in many parts of Warsaw two weeks ago. The transmission is scheduled for Friday, the eve of May Day.

The inauguration of "Radio Solidarity" was a major embarrassment for the government, which seemed unable to trace the transmitter. Several radio-detection vans have been seen in the streets of Warsaw recently in an apparent attempt to track down the source of the broadcasts.