The health ministry initially said yesterday that one child had been among the victims but later corrected its report and said the four fatalities were adults. The youngest victim was 20.
The ministry updated the number of people injured to 26, with three suffering serious injuries.
The bus was transporting the group from the northeastern city of Iasi to the western city of Arad for the festival.
It overturned near the village of Persani in central Romania. Local police spokesman Liviu Naghi said the bus hit a concrete barrier in the middle of the road as it went around a bend, and overturned.
Eleven ambulances were transporting the injured to hospitals.