May I ask why your system doesn't allow usernames to be less than four characters?

For the purpose of ensuring that new members create valid usernames.

What is invalid about a three letter username? How is that less valid than abcdxyz?

JanS, most would start using acronyms or initials, instead of full featured names. Knowing and communicating with a somewhat full name is better than with initials.

Best,

Sorry Art I have to play the devils advocate here ....sooo...

Unless the name was Joe...Jan....Al...etc... .

True. In that case you either add an initial, or you think of another name.
The number of users who would abuse the 3 character names will be greater than the number of users with real 3 character names.