Job decisions are based on four criteria: success, loyalty, reputation and experience. Also, schools prefer coaches who are actively coaching over those that are unemployed. Success is based on the four most recent seasons and the more recent the season the more weight it carries. Though not exact you can get an idea as to what your success was for a season by looking at the WIS Ranking for that season. The coach with the best resume based on these criteria will be hired.
What's the reasoning behind only using 4 seasons?
The reasoning is that coaches are very much defined by a "what have you done lately" view in real life and we wanted to carry that over into the game. We have to do this in order to give coaches with less longevity a chance at jobs. If we didn't "expire" accomplishments the highest rated coaches in the game would be the coaches who have been around the longest and not necessarily those that are winning now.