Age
I agree that age sounds like a more reasonable way to determine seniority, but that is just an opinion. I think Sandman makes a reasonable explanation of why a random number generator (the last 4 of the SSN) is more objective, but the bottom line is the way FedEx DOES it is by the last 4 of the SSN. I wouldn't worry too much about the seniority within a class, its kind of like a bunch or hookers sitting around debating their virtue.
I was lucky with my last 4, I gained a couple of numbers from what I would have been if they used age, but either way I was just happy to be there (and we were stuck at the bottom for the entire year and a half hiring freeze). During newhire training you do have to go on bathroom breaks in seniority order, so if you have a small bladder it might be important to be up near the top.
Ok, that last part was a joke. Seriously though, you do pick your training partner and sim locale based on seniority within the class which is cool. Otherwise, with hiring continuing on pace like it is now, seniority within the class isn't much of a factor. That being said, of course I'd rather be at the top than the bottom, every little number helps.
Good luck in class and hurry up and hit the line before Christmas please!
FJ