Iraqi police say that at least 17 people in total were killed in a series of bomb attacks and shootings across the country on Friday.
The four Shia pilgrims were killed as tens of thousands of Shias travelled to Karbala ahead of a major religious festival on Saturday.
The attack took place near the town of Musayyib, 55km (34 miles) south of Baghdad.
The mortar attack took place despite heavy security.
To prevent attacks Iraqi soldiers have cordonned off the city of Karbala, while nearly 7,000 policemen guarded the Shia shrines there.
Previous Shia festivals have been targeted by Sunni fighters using mortars and carbombs to inflict heavy casualties.
Baghdad police targeted
In a separate incident, Colonel Ali Muhammad, head of the police for Baghdad's Karkh district, was the target of an apparent assassination attempt.
His convoy was hit by a car bomb as it passed through the Zayuna district of central Baghdad.
While Muhammad escaped unhurt, one person was killed and six others, including three policemen, were wounded.
In another attack near the northern oil-city of Kirkuk, armed men killed Shaikh Khalaf Ibrahim of the Sunni Juburi tribe, a police captain said.
The attack took place in the Assad al-Baggra village, close to Hawija.
In another incident near the village, armed men clashed with US troops "in which two gunmen were killed", police said.
The men also attacked an Iraqi army patrol between the town of Samarra and Baghdad, killing a soldier and wounding two.
Meanwhile, police in Tikrit - the hometown of Saddam Hussein - said they found the bodies of three members of the oil protection force who were kidnapped a day ago from west of the city.
In a similar incident, police in the central city of Diwaniya found the body of a translator who worked for the US forces. He was kidnapped three days ago.
Police also found the bodies of six men who had been tortured and then shot and killed in an apparent sectarian killing in Baghdad.