BONN, West Germany (AP) _ Prosecutors said Thursday a senior diplomat was arrested as a spy, the fourth government employee seized this month and accused of providing secrets to East Germany's former Communist rulers.

The 54-year-old suspect, indentified only by the initials H.B., spied for East German intelligence since 1961 and is believed to have passed on information while posted at embassies in Cairo, Tokyo, London and Vienna, said spokesman Rolf Hannich of the chief federal prosecutor's office in Karlsruhe.

Hannich said the diplomat, whose rank was not revealed, was said to have furnished ''vast amounts of information'' to East Germany before being arrested Wednesday.

Hannich said the Constitutional Protection Office, the nation's highest spy-fighting unit, helped uncover the diplomat's activities.

West German newspapers said changes in East Germany have helped West Germany's counter-intelligence efforts. News media say West German spy- fighters have been receiving useful tips from former East German agents.

A week earlier, Hannich announced the arrests of two other West German government employees suspected of spying for East Germany - another Foreign Ministry employee and a high-ranking official for the federal border police.

On April 5, the federal prosecutor's office announced the arrest of a 45- year-old man working for the West German NATO mission in Brussels.

Spiegel magazine identified that suspect as Heinz-Helmuth Werner, a former West German soldier, and said he delivered a bonanza of classified information to the East Germans and the Soviet KGB.

In Ankara, the capital of Turkey, the Turkish government on Thursday decided to recall 15 Turkish diplomats from West Germany, following allegations in the West German press they engaged in espionage.

Turkey also asked West Germany to remove eight of its diplomats from Turkey.