The winner of the playoffs is automatically promoted.
In addition, any bot teams are automatically demoted and replaced with teams in the lower divisions on the basis of regular season record, regardless of group. So finishing first in your group will not ensure you get bot promoted, but if you finish 21-1 there is a pretty good chance you will get promoted.
Second, I don't understand why you wouldn't want to be promoted? It's a lot more fun playing against fewer bots and more players who have an idea of how the game works and there is WAY more money to be made in division three than in division four (i.e. 100%-200% more).
What's the point in playing the game if you're not trying to win? If you are just going to throw games at the end of the season, you may as well just throw every game at the beginning of the season straight to the end to get better draft picks.