I've selected "quizzical" as a hard difficulty word. It's 9 letters long (within the 4-10 limit), contains uncommon letters like 'q' and 'z', and has a double 'z'. While it's a real English adjective that means "suggesting puzzlement," it's not commonly used in everyday conversation. The presence of the letter 'q' at the beginning and the unusual letter pattern makes it challenging for the opponent to guess, but it remains fair as it's a legitimate English word without any proper nouns, acronyms, or special characters.