I was wondering about that. But I could see that database getting filled rather quickly. I would imagine that sometime in the future there would have to be a time limit on the links.

Though they do have a lot of codes. You're right, even 4 digits would offer a lot, since they can use numbers, letters, upper and lower cases, and I assume that something like "0040" would be different from "004", so that gives them even more room.

Still, I would think that after a while, they'll need an expiration for the links.