The plus four changes all the time so it isn't feasable to know it. The use is large mailers can get a discount by looking it up and presorting mail. If the mail coming into my post office has my mail next to my next door neighbors that saves them a lot of time.

Is that still true? I would imagine any reasonably modern computer could map every physical address in a huge region to a (route number, stop number) pair. I wouldn't think the +4 would add a lot of value anymore.

The sort everything outgoing by where it goes on the truck is valuable. sure computers can sort but this is physical things and so mechanical limits apply.