The only team that is guaranteed promotion is the league playoff champion.
But after the regular promotion/relegation, there is a bot cleanup where additional teams from a division can promote. From D.IV this can include promotion to replace bots relegated from D.II; bots that would have remained in D.III (such as a 5th place team); and bots that promoted to D.III (only about 3/5 of USA D.IV leagues are full).
The extra teams that promote are selected based on their regular season record: (1) conference place, (2) games won, (3) point difference, (4) points scored.
In the past, this extra promotion has extended down several places, but this season only 1st place teams with pretty good records were promoted. LA promoted as a 1st place team with a 19-3 record.
While 18-4 might have been enough to get promotion through the cleanup, your 3rd place finish would have stopped it.