Re: 4 character extensions: these are made by python-social-auth and while the length is easy to change, the character set is not. See https://github.com/omab/python-social-auth/blob/21f462f18dd1b5917e811ab084e3470ce9969aa5/social/pipeline/user.py#L9.

We could write our own implementation of get_username, but it didn't seem high enough value for v1.