FWIW: Last year I tried to check a bag at SEA about 4 hours and 15 minutes before the flight and the nice agent suggested I wait until the 4 hour mark. She said that something happens at the 4 hour mark that makes it more likely my bag would get to the proper destination at the proper time. Maybe a batch is opened at that time? Wish I could remember the exact words but I can't. In any case I waited to check.
On the other hand we got to HNL really early one time and the agent had no problem checking the bags before the 4 hour mark. She said it wasn't a problem.
If someone understands the mechanics of all this please share.