I've chosen "zenith" as a medium difficulty word. It has 6 letters, which falls within the required 4-10 letter range. It contains only English letters and is a real English word meaning "the highest point" or "peak." It's not a proper noun, acronym, or made-up word, and contains no spaces or special characters. This word presents a moderate challenge because it contains the less common letter 'z' and has a somewhat uncommon letter pattern, but it's not so obscure that most people wouldn't recognize it.