I've chosen "quizzical" as a hard difficulty word. It's 9 letters long, contains uncommon letters (q and z), and follows an unusual spelling pattern. The word is a legitimate English adjective meaning "suggesting puzzlement" or "expressing confusion." It doesn't violate any of the rules - it's between 4-10 letters, uses only standard English letters, isn't a proper noun or acronym, and contains no spaces or special characters. The uncommon letter combination and less frequently used word will make this a challenging but fair word for an experienced Hangman player.