Arena Football, also known as Indoor Football was developed in the 1980s as an 8-man form of Indoor Football. After an initial few seasons where the game was developed with a 4 team league, a full seasons League with eventually over a dozen teams was developed in the 1990s which was considered by the Wall Street Journal to be a fifth major.
The Arena Football League overexpanded in the 2000s with a Minor League, and its importance (and number of teams) waned, but there is now Arena Leagues all over America, all of which would now be considered Minor League or Semi-Pro.
Arena Football League (Major/Minor National League):
Minor Arena Football Leagues (Overviews):