Foreign Minister Kamal Kharrazi arrived in Lebanon on Thursday for a two-day visit during which he will meet top Lebanese and Hizb Allah officials.
The diplomats went missing in Lebanon in 1982 during the civil war and Israeli invasion of that year. Tehran believes the diplomats, reportedly kidnapped by the Lebanese Forces (LF) militia then allied to Israel, are alive and possibly in Israel.
Lebanese militia leaders have since said the four were killed shortly after they were captured.
Israel and the Lebanese resistance group carried out a historic German-mediated swap last week that freed hundreds of Arab prisoners for a suspected Mossad spy and the bodies of three dead soldiers.
The first stage of the swap included two prominent Hizb Allah leaders, Shaikh Abd al-Karim Ubaid and Mustafa Dirani, snatched from their homes in Lebanon by Israeli commandos in 1989 and 1994 respectively.
They were held as bargaining chips for missing Israeli airman Ron Arad whose plane was shot down over Lebanon in 1986 during the war.
The second phase of the talks will focus on securing the release of the longest held Lebanese prisoner Samir al-Qantar in Israel and information on the Iranian diplomats in exchange for concrete evidence on the fate of Arad.
Israel alleges that Lebanese fighters handed Arad over to their Iranian backers and that he might still be alive.
Relatives of four missing Iranian
diplomats visit mission in Beirut
But Hizb Allah Secretary General Sayyid Hasan Nasr Allah has said that Arad is not in Tehran. The Islamic Republic also says it has no information on his fate.
Germany will form separate committees with Israel and Hizb Allah to resolve the outstanding issues.
"We hope that the fate of the four Iranian diplomats who were captured would be determined through our talks today and tomorrow and through the committee that will be formed," said Kharrazi.
He said there was "a lot of information" that the four were in Israel but gave no details.
Lebanese Forces responsible?
In Israel, a senior security source said the diplomats were "murdered" by the LF militia "in what appears to be a localised settling of scores".
"This was a crime in which we had no part. We have made this clear to the other side repeatedly, through various channels. Western intelligence agencies corroborated our account," said the source.
The source said the Iranians were still reluctant to believe that they already have all the facts.
Raid al-Musawy, son of one of the missing diplomats, said the families believed they were still alive and held in Israel.
"The information available to us shows they are alive and we are certain of that...They are alive and held in Israel," he said, adding that some in Israel wanted to exchange them with Arad.