I would expect any player placed on IR to (1) have an injury sufficient to limit their value to the team this season, and (2) going on IR to rehab, not to "redshirt" them. Out the four you listed, one is currently practicing with full participation and the other three have been rehabbing since Cincinnati nearly two weeks ago - sufficient time to decide if they could come back or not. They also happen to be three whom I'd expect Bill to want on his roster for their development, IR'ing them doesn't make much sense short of a season ending injury.