The four were only allowed to postpone enlistment until the end of 2023.
RM, Jimin, V and Jungkook will likely be called later this year or early in 2024.
Before them, Jin, J-Hope, and Suga begun their military service.
South Korean men are required to serve for 18 to 21 months.
BTS has been on hiatus for a year now.
The band is expected to stay on hiatus till 2025.
Meanwhile, all 7 BTS members have entered Billboard charts with solo albums.
BTS's documentary Monuments: Beyond The Star will release on December 20.
This docu-series showcases the group's journey in 10 years.
After completing military service, BTS hopes to reunite in 2025.