The four children included two siblings Hoang Thi Mai, 13, and Hoang Thi Huong, 11, as well as 13-year-olds Ly Thi Tham and Nguyen Thi Hong Hanh.
The victims' families said they had taken cashew nuts at about 11am and then went for a bath in the reservoir.
A farmer saw that the children were in trouble, but it was difficult to save them due to the deep water. Their bodies were found at 4.30pm.
Local residents said a few children had drowned in the 10ha reservoir earlier.
VNS