I've chosen "zephyr" as a medium difficulty word. It's a 6-letter word (between the required 4-10 letters) that means "a gentle, mild breeze." This word contains common letters like "e" and "r" but also has less common letters like "z" and "y" which makes it moderately challenging. It's a legitimate English word (not a proper noun or acronym), contains only letters a-z, and has no spaces or special characters. The mixture of common and uncommon letters provides a good balance for an engaging Hangman game without being frustratingly difficult.