I've chosen "quantum" as a medium difficulty word. It's 7 letters long (within the 4-10 range) and contains a mix of common vowels (u, a) and less common consonants (q). The word follows a somewhat unpredictable letter pattern, making it challenging but not impossible. It's a legitimate English word that doesn't violate any of the stated rules - it's not a proper noun, acronym, and contains no spaces or special characters. The presence of 'q' at the beginning will likely be a surprising first guess for many players, adding to the challenge.