Why do inmates get transferred from level 4 prisons to level 3 prisons or lower?

As an inmate does his time and is not receiving any incident reports, their custody level will change for the better. That is the incentive for an inmate to maintain good behavior. The lower the level of custody, the easier their time is. The higher security facilities have inmates with long sentences and not much to lose when it comes to igniting situations with rule breaking or violence. Inmates that want to go home follow the rules and get rewarded with better housing units, units with more recreation, etc. The perks of prison.