ORAN, Algeria -- The world's big oil producers, flush and powerful just months ago, said they would cut crude supplies by a record amount but found even that couldn't stop prices from sliding to their lowest level in four years.

The plunge in oil prices has slammed oil-producing economies and shelved important energy projects. Oil has fallen by more than two-thirds from its high last July, turning a boom year into one of the most alarming in memory for producers. On Wednesday, OPEC said it would cut output by 2.2 million barrels...