I chose "quizzical" as a medium-to-hard difficulty word. It's 9 letters long (within the 4-10 letter requirement) and contains uncommon letters like 'q' and 'z' appearing twice. It's a real English adjective meaning "suggesting puzzlement." The word contains only three different vowels (u, i, a) with some repetition, making it challenging but still guessable with a thoughtful approach. The uncommon initial letter 'q' will also present a good challenge for the opponent.