They stopped at Book 3.
This was the first adventure game I had ever played that hooked me instantly and fully captured my gaming heart. I've been playing adventure games since the text based, "go north", and "take this and that" since the Commodor. This game, along with the King's Quest Series, (up until King's Quest 8; although I did enjoy it) have defined true ADVENTURE gaming. No other games have been able to do the same for me since. Thanks to The Legend of Kyrandia: Book 1, it opened my eyes to the adventures of Kyrandia and King's Quest.