Yugoslav police have arrested four of West Germany's most wanted terrorists suspects, giving West German authorities their biggest and most important success since launching a massive Europe-wide dragnet 10 months ago after the kidnap-murder of industrialist Hanns-Martin Schleyer.

The arrests are important not only because of the captured suspects - two of whom are among the 15 most wanted hard-core West German terrorists - but because it also indicates that cooperation in apprehending urban guerrillas who move across international borders is improving.

In September, 1976, the most wanted international terrorist, the Venezuelan known as Carlos, was known to be in Belgrade but was quietly allowed to leave by Yugoslav authorities, a move that privately outraged U.S., French, West German and Austrian officials.

Carlos, however, has strong ties to Palestinian guerrilla organizations and some radical Arab countries and the Yugoslavs apparently did not want to damage their relations with Arab and Third World countries.

Officials of the West German Justice Ministry could provide no details of the terrorists' arrests. They said that the government had asked Belgrade to extradite the four.

They also disclosed that the Yugoslavs recently requested the extradition of two Croatians serving prison terms for the attempted murder of a Yugoslav official here last June. They said, however, that Belgrade was not insisting on getting the Croations in exchange for the terrorist suspects.

The news of the arrest of the four Germans emerged under curious circumstances, suggesting that Belgrade still is not looking for any public credit. The four suspects, according to the Justice Ministry officials, were arrested last week and the announcement came as a result of inquiry at a routine ministry press conference here yesterday.

Top West German officials said privately that the Yugoslavs were still sensitive about the matter but added that cooperation between West German security forces, who helped trace and identify the suspects, and the Yugoslavs was good during the manhunt.

These officials indicated that the most important factor leading to the arrests was the capture by French police in Paris' Orly airport two weeks ago of another West German terrorist suspect, Stefan Winsniewski. He was taken into custody after he landed in France aboard a plane from Zagreb, Yugoslav.

Officlas here have long suspected that West German terrorists, most of whom are believed to have scattered to other countries after the Schleyer killing, try to get back into central Europe and eventually into West Germany from East European countries. Although Yugoslavia is a Communist country, it is outside the Soviet bloc.

The two major suspects now in Yugoslav custody are Rolf-Clements Wagner, 33, and Brigitte Mohnhaupt, 28. Both are among the 15 terrorists most wanted in connection with the slayings of Schleyer, Chief Federal Prosecutor Siegfried Buback and banker Juergen Ponto, all of whom were killed last year.

Wagner is suspected of playing a direct role in the kidnaping of Schleyer last September, a move that cost the lives of three of the industrialist's bodyguards and his driver and eventually led to the hijacking of a Lufthansa airliner and the murder of Schleyer in October.

Mohnhaupt, like Wagner, is suspected of membership in West Germany's Red Army Faction, an offshoot of the Baader-Meinhof terrorist gang. She narrowly escaped arrest last September in a shoot-out with Dutch police in which one policeman was killed and another West German terrorist, Knut Folkerts, was arrested.

The Bonn government has asked for extradition of these two plus Sieglinde Hoffmann, 33, and Peter Broock, 26. Hoffmann may have been a key figure in tracking down the four since it is believed she received training in guerrilla warfare at a Palestinian camp in South Yement along with Wisniewski, who was arrested this month at Orly.

Boock is the husband of another jailed terrorist, Waltraud Boock. Both Hoffmann and Boock are suspected of belonging to another off-shoot of the original Baader-Meinhof gang built around radical lawyer Siegfried Haag. Haag is also in jail facing charges of being an accomplice to murder in the attack on the West German Embassy in Sweden a few years ago.

The arrests could not have come at a better time for West German police authorities who, despite a massive investment in computers and the latest in crime-fighting technology, have managed to capture only a handful of terrorists out of the 40 who are most wanted. This has subjected the police to some ridicule, even though the problem largely has been that most of the terrorists have probably fled West Germany and catching them depends on the help of other countries.

This weekend, Bonn was embarrassed by the successful jail break in West Berlin in which two heavily armed women slipped into a supposedly top security prison and freed suspected terrorist Till Meyer.

West German police, it has been learned, actually expected some major new terrorist action in recent weeks. "They weren't prepared for anything so siimple as the West Berlin prison break," one official lamented.