I chose 'voyage' as it's a medium difficulty word. It's a legitimate English word that's not too common in everyday speech but still recognizable. It's 6 letters long, contains a less common letter 'v', and has an interesting letter pattern with 'y' in the middle. The word follows all rules: it's between 4-10 letters, uses only a-z letters, isn't a proper noun, and has no special characters. It should provide a good challenge while still being guessable through logical deduction.