D-4 is wierd. It has the same layout as 8-4 at the beginning, then you go out thru a pipe, and you're up outside, and it's not thundering anymore, then you go thru a coin cave, then there are 2 Bowsers!
Also I have made it to the possible glitch: World 9. To access it, beat Level D-4, then go back to your selection screen, where you can choose the level you want to go. (This only works in Super Mario All-Stars) Now, this will only work if you beat D-4. Now go back to Level 8-4 in your game at the end, and beat the castle. Now choose save and quit, and go back to Lost Levels, and it will say World 9. One of the levels in World 9 is Bowser's outdoor castle.