Basically, if you miss the playoffs then you play a 3 game series to stay in your league. There are two 5v7 relegation series and one 6v6 relegation series. The division system work to where you either move up by winning the league or in a rare case where a CPU team replaces a team at a higher level (you almost never see one at the III level, and I absolutely have never seen one at the II or NBBA level), your team gets chosen (not sure what the process is) to replace them, stay put by either not winning the league or by winning a relegation series, or you move down by either losing a relegation series or finishing last in your division. Usually, if you move down it would be a league down(for example, from IV to V) but in your case, I think you ended up replacing a would be CPU team because the people running the game want as many people run teams in the higher level leagues in this game as possible, so after having just joined they let you stay in a IV level league instead of putting a CPU team in over your team.