I've only stayed at one Four Seasons (Tokyo), but that sort of "only $100 more" than a more moderate hotel is why I did it, and I was not disapointed. It's worth it for the experience. The only reason not to go for it is if you think you'll get too spoiled! You don't mention the actual price that it's $100 different from, but it seems like it would have to be a pretty darned good deal for the FS particularly during popular event. Now is the time to treat yourself. Jump on it before it's sold out.