The questioning on Saturday along with a decision on whether to release them or issue arrest warrants was expected by the end of the day.
The interrogations continued even as Syria on Saturday announced it was ready to receive the head of the UN commission into the murder of al-Hariri, officials in Damascus said on Saturday, adding that no date had been fixed for the visit.
"Syria is ready to receive the head of the international commission of enquiry into the assassination of former prime
minister Rafiq al-Hariri but no date has been fixed for the meeting," a Syrian official said.
In Beirut, magistrate Ilias Eid on Saturday first questioned the commander of the Presidential Guards Brigade, Brigadier-General Mustafa Hamdan in the presence of his lawyer, and judicial officials said he would then call the former chief of general security, Major-General Jamil Sayyid.
The suspects are being interrogated at Beirut's Justice Ministry building under tight security.
Eid on Friday questioned two other suspects - the former director general of military intelligence, Brigadier-General Raymond Azar, and the former director general of the Internal Security Forces, Major-General Ali al-Haj.
The four generals have been in detention since Tuesday when the prime minister announced that the UN investigators into al-Hariri's 14 February murder had named them as suspects.
On Thursday, the chief UN investigator leading the al-Hariri assassination inquiry, Detlev Mehlis, said he believed the men played a role in planning the murder.
Mehlis believes the four suspects
played a role in al-Hariri's murder
Later that day, Lebanon's top prosecutor issued preliminary criminal charges against them.
Under their detention orders, Eid has until Saturday night to decide whether the four should be released, remain in custody or face arrest warrants.
On Friday, the UN investigators interviewed for five hours former labour minister Asim Kansu, who is also the head of Syria's Baath Party branch in Lebanon.
Kansu told newspapers later he was summoned as a witness and questioned about his relationship with the four detained suspects and former Syrians security chiefs in Lebanon.
Meanwhile, Mehlis left Lebanon for Geneva early on Saturday on his way to the US where he is scheduled to meet UN Secretary-General Kofi Annan to brief him on the progress in the investigation.