Even though the USA has a 5th division, there are no human-owned teams in it, and so teams from D.IV don't relegate, even if you finished 8th. The 6th and 7th place teams also actually play the relegation games, though neither will relegate.
Also there may be some additional promotions to replace bots in upper divisions. In the past these have been based on the regular season finish and record. So if you finish in the top 4 in your conference, and then lose in the playoffs, you might still promote. You will know if your team shows up in D.III.