UPDATE 4-Iraq's largest oil refinery shut by bombing

* Baiji refinery attacked, four people killed

* Damage severe, will take days to fix

* Small Samawa refinery also shut after fire

(Adds second refinery shutdown, paragraphs 2, 13-14)

By Sabah al-Bazee

BAIJI, Iraq, Feb 26 (Reuters) - Militants attacked Iraq's largest oil refinery on Saturday, killing four workers and detonating bombs that touched off a raging fire and shut down the plant in northern Iraq, officials said.

In the southern town of Samawa, a second refinery was shut down by fire but officials said initial reports indicated it was started by a technical failure rather than an insurgent attack.

The militants planted explosives at a kerosene and benzene production unit at the northern refinery in the town of Baiji, a former al Qaeda stronghold 180 km (112 miles) north of Baghdad, the governor of Salahuddin province, Ahmed al-Jubouri, said.

"The refinery has completely stopped," Jubouri told Reuters. "It's a big loss for the whole country. All Iraqi cities depend on its production."