Game 1: Fairly obvious that Algie was not feeling well (plus he almost lost the ball on the first play of the game and dropped a pass that CB was ready to crucify him for), Hine was still the same unproven Hine that all of CB was saying could never run between the tackles. So Carter gets carries. There is no fourth string.
Game 2: Hine establishes himself as the man for the '15 season. Brown gets hurt. Carter has 2 runs, one before and after Brown gets hurt. Clearly the third back.
Game 3: On the road, Bernard is getting healthy but clearly do not want to use him yet (shown by the fact that Langi was short yardage back). No brown. Hine gets bulk of carries. Carter gets 7 carries for 22 yards. Who else did you want the coaches to use??
Game 4: Brown is out, Hine gets hurt, Carter gets 3 carries for 9 yards. BYU now has two RB's Bernard and Carter.
Game 5: Brown appears healthy, Bernard is second in carries. Carter is third in carries (8 for 14). Riley has the greatest 5 yard run in BYU history...but several reports claim ball control.

When you look at the injury history, the only game to really question the carries was last game. Even then, it is clear that Carter has been passed by Bernard in just two quarters of play. In games 1-4 there was no one else to carry the ball. NO ONE. Carter was second string with Langi in one game as third string. The decision to play Carter was not a talent decision, it was decision out of necessity. Something depth has forced them to do. The cupboard was completely bare.

Rest assured CB, Carter is now the third string running back and when Hine comes back, he will be fourth string and will not be getting any carries at all. Then we can move on from bashing the guy that CB was clamoring to get more PT since the Nevada game of last year.