I've chosen "whisper" as a medium difficulty word. At 7 letters long, it falls within the required 4-10 letter range. The word contains the less common letters 'w' and 'h', which makes it somewhat challenging to guess, but it's still a familiar English word that most people know. It has no proper nouns, acronyms, spaces, hyphens, or special characters - just regular a-z letters. This should provide a fair challenge without being too easy or too frustrating.