I have two characters ready. Is that not enough? Can you only play the game with four? The message appears after two characters already but pressing Enter does nothing.
I can confirm that the Enter key does nothing. This is on both Linux and Windows and on the same machine (a new latop I bought last summer). I will try on my macBook tomorrow. There is no other issue with the Enter key anywhere on Linux and/or Windows in any app so this is as WTF as it gets, I guess. :)