Shortner method is terrible, will result in collisions and will always be 4 chars.

[firepacket]

hash = hasha(url, {encoding:"base64", algorithm:"md5"}); hash = hash.slice(0,4);

If you're going to make a site like this at least find out how to do it properly.
Your first few thousand links should be 2 chars, and go up slowly from there with zero collisions.
(1/36)*4 = 11% collisions.