Two of the boats were about to sink when they were found south of the maritime border near the South Korean island of Baengnyeong, and the four crewmen were scooping water out of the vessels, according to the official from the South's Joint Chiefs of Staff.
The four men are currently being investigated at relevant organizations, the official said on the condition of anonymity.
A third fishing vessel was found drifting in waters near Baengnyeong later the same day, but its three crewmen were sent back after being supplied with fuel.
"We verified that the crew we sent back had no intentions of defecting and (our policy is to) return fishing vessels that do not trespass intentionally on humanitarian grounds," Yonhap news agency quoted the official as saying.
"For now, it isn't clear why several boats crossed the border (simultaneously)," the official added.