The accident happened in the early hours in Villacanas, a village in the Castilla-La Mancha region of central Spain.
The ride consists of four large metal arms that spin around an axis, and cars at the end that also twirl.
One of those four arms snapped in two, said Jose Luis Gonzalez Capilla, a spokesman for the Civil Guard police force in the regional capital Toledo.
Those who died were three Romanian men in their 20s and 30s, and a 16-year-old Romanian girl was severely injured, Gonzalez Capilla said.
Many Spanish towns and villages hold summer festivals or carnivals. The tragedy occurred on the first night of the fiesta in Villacanas, home to 9,000 people and located about 80 kilometers (50 miles) east of Toledo