I just finished King's Quest IV. I liked the storyline.
I enjoyed Rosella as a character, and didn't find her to be as undefined as people often say she was. She's portrayed as being musically gifted, strong-willed, loving, and a bit of a free spirit (which was strongly hinted in her dialog at the end). I enjoyed the day and night aspect of the game. The same scenes were so creepy repainted darker at night, especially when they were populated with zombies and trees with frightening faces. The thing I didn't like about this game was the thing I always have trouble with in Sierra's early games, especially those darn stairs. The stairs that just go left and right now thankfully just let you walk up them with just the right arrow key, but the twisting stairwells are still so very annoying. It's even more annoying when only some parts of the stairwells have railings! The artwork of the twisting stairwells looks nice, and I know that's the reason Sierra always put them in their early games, but they're just frustrating when you have to change your arrow movement based on perspective. It's annoying to have to save with every step you make just because you're battling the interface. But, this was part of the Sierra game design philosophy back then, and one of the main reasons LucasArts decided to make theirs vastly different. If you can tolerate the old Sierra interface, the game is worth playing.