As in last week's attack, the bombing in the port city of Dammam targeted Shiite worshipers at Friday prayers, the most important of the Muslim week. But unlike the earlier strike, also in Saudi Arabia's east, the bomber did not manage to make his way inside the mosque.
The previous attack left 21 dead, while Friday's death toll was considerably lower.
It was not immediately clear whether the four fatalities reported included the assailant. Four other people were injured, according to officials cited in news reports.
The Islamic State group, which has taken over large parts of Iraq and Syria, views Shiites as apostates and has urged its followers in Saudi Arabia to carry out attacks against them, presenting what could become a major security challenge to the conservative Sunni Muslim-dominated kingdom's recently reshaped leadership.
Last week's bombing was the first on Saudi soil for which the group had formally claimed responsibility, though Saudi authorities had suspected Islamic State in previous attacks.
The official Saudi Press Agency reported that the attacker in Dammam was stopped by guards outside the mosque, saying that authorities had "managed to foil a terrorist crime." The Associated Press cited an unidentified security official as saying the bomber had been disguised as a woman, in a long black robe and face-concealing veil.
Eastern Saudi Arabia is home to many members of the kingdom's Shiite minority. Tensions have simmered for years between Shiites and the government but have been heightened lately as the Saudi leadership has moved aggressively to counter what it sees as expansionist ambitions by Iran, the region's Shiite power.
The two countries are engaged in what amounts to a proxy war in Saudi Arabia's neighbor, Yemen, where Shiite rebels have overrun much of the country and a Saudi-led coalition has carried out more than two months of air strikes.
Fighting and bombardment in Yemen have killed at least 2,000 people, by international aid groups' estimates, and caused widespread suffering amid shortages of virtually all essentials. The Saudi-led coalition has blockaded Yemen's ports and airports, accusing Iran of funneling weapons to the Houthi rebels.
The claim of responsibility in Friday's bombing, reported by the SITE Intelligence Group, said the attack was carried out by an Islamic State-affiliated group calling itself Najd Province. As in its previous claim, the group identified the bomber by name and accused worshipers at the mosque of being apostates protected by "Saudi tyrants."
Staff writer King reported from Alexandria and special correspondent Hassan from Berlin.