Yes, you are correct. I don't have anything fancy planned. Users with accounts are able to customize the ending so this will increase the availability. Currently, Bitly is up to 7 characters long when you create a random short URL. T.LY is already 2 characters shorter and there are still plenty of short random URLs 62^5 = 916,132,832
years ago, we asked bit.ly if they could vendor our link shortening and tracking. Turned out we were already a couple multiples larger than them in terms of link processing. our use case is a bit different though. Instead of one link given to many, we tend to be one link given to one recipient. Optimizing this kind of problem is interesting and fun.
What's your plan once you reach the 4-character limit? Roll over to 5, or something fancier?