Two people were killed in protests, then two more died when gunfire broke out during a funeral procession, Interior Ministry spokesman Mansur al-Turki said.
The fighting occurred in the Qatif region of Eastern Province, where many Shiites in the predominantly Sunni country live.
Al-Turki blamed "unknown criminal sources" for the casualties.
"The goal of the rioters is to achieve suspicious objectives dictated to them by their masters abroad," al-Turki said. He did not specify to which country he was referring.
There have been street disturbances in nearby Bahrain, a predominantly Shiite country ruled by a Sunni monarchy. And there has been tension between Iran, also a predominantly Shiite nation, and the Sunni world.
Earlier this year, Shiite protesters in the Qatif region urged the Saudi government to release Shiite prisoners and grant more rights. Those protests, which were peaceful, also urged the withdrawal of regional forces from Bahrain.
Al-Turki said Thursday's violence in Qatif "started gradually ... and it escalated with what happened today
And they blame Iran and Syria
