T4, is the Fourth Thaileague 2 League in Thailand. In 2009, it was divided into five groups by geographic region.
Each group contained 16 clubs except for the Southern Region group which contained 13 clubs. It was sponsored by AIS and therefore officially known as the AIS Regional League Division 2. The 3rd Level League was formed in 2006 with 10 member clubs. The league was created so that clubs from regional leagues could come together and play in a national league with the onus being on promotion and relegation so clubs could easily move up the Thai football system ladder depending on performances on the football pitch rather than performances behind the scenes. The league was also created so it would combat against the rival Provincial League.
With future growth moving the Pro League to join the 2nd Division. The league suffered another contraction after 2016 season, as 31 teams joined the newly created Thai League Championship. It also moved a tier down the pyramid, making it 4th Level League since 2017. In 2006, the first season, 10 clubs played each other twice, with promotion going to the championship winner Chula-Sinthana FC.
In 2008, although two clubs were relegated at the end of the 2007 season, the league was again expanded the following year, to 22 clubs. 11 clubs in Group A and 11 clubs in Group B. At the end of the season, 2 clubs from each group would be promoted with the league receiving four relegated teams from the Thailand Division 1 League. At the end of the season, the four clubs who achieved promotion would play off in an end of year competition to determine the overall league winner. No clubs would be relegated due to re-structuring of the league system again for the 2009 season. But all clubs who wish to play at this level were asked to re-submit to the league and each club would be graded such items as club structure and ground stadium. Each club was expected to have certain criteria in place to achieve league status.
In 2009, 52 clubs played Division 2 League was combined with Provincial League by Football Association of Thailand. Central East Zone, Bangkok central zone and South of Thailand. 75 teams were confirmed to play in the Regional League for the 2010 season. Four teams would automatically be promoted to Division 1. The four promoted teams would be decided by two mini leagues at the conclusion of the regular season. The total number of teams competing in the mini leagues will be 12, they will be made up of the top two from each league with the best two 3rd placed teams from the Northern, North-east and Central-East leagues The top two from each mini league will gain promotion. 94 teams were confirmed to play in the Regional League for the 2016 season.