http://www.espn.com/college-football/story/_/id/23861759/what-does-four-game-redshirt-rule-mean-division-teams
from the article just a few ways the 4 games will be used ....
Roster Management - holding guys back to cover for late season injuries
Motivation - maintaining incentive to practice hard and work in the weight and class room
Transfer Deterent - can't recruit the 5-star QB and not play him