Popular German bank „Sparkasse“ has this great online banking system that allows you only 5 characters (Small and large, numbers, umlauts and eszet[1]). They call it a PIN (Personal identification /number/), but IMO its terribly insecure.
5 characters with those options obviously isn't enough, but it's still better than "four digit numeric", which is also out there. That's such a small space that you can try it by hand, for god's sake.
Feel free to submit a PR if you like.