This article has a talk page!This is a sub-page of Sonic the Hedgehog 4: Episode II.
As Sonic the Hedgehog 4: Episode II was built using Episode I as a base, almost every Episode I level (the exceptions being E.G.G. Station and the Special Stages) remains in the files. Act 1 of every Zone is used in Episode Metal; however, the rest are left unused. Most levels crash almost instantly due to uninitialized objects (either invalid IDs or nonexistent 3D NN data).
(Source: RadiantDerg)
Splash Hill Act 2: High-Speed Athletics#
This is one of the two levels that does not crash at any point, and it is the only level that can be completed.
The vines and the corkscrews do not work as intended. You can swing on the vines like in Episode 1, but Metal Sonic doesn't have a swinging animation and uses his idle animation. The vines also launch you much higher than intended. 
The corkscrew makes you go through it in a straight line. You can still fall of with low enough speed, just like in Episode 1. 
There are also a few spots where the background disappears. 
Splash Hill Act 3: Sunset Dash#
This level is missing the sunset lighting that is in Episode 1. The level also crashes almost immediately due to the missing zip-line object.
Splash Hill Boss: Showdown with Dr. Eggman#
This level crashes immediately.
Casino Street Act 2: Road of Cards#
The dice work as intended, but they play no sound effects when interacted with.
The level crashes almost immediately because of the missing cards.
Casino Street Act 3: Casino Climax#
This level crashes almost immediately because of the missing cannon object. 
Casino Street Boss: Dr. Eggman's Party#
This level crashes immediately.
Lost Labyrinth Act 2: World of Darkness#
This level crashes immediately.
Lost Labyrinth Act 3: Underwater Maze Escape#
This Level does not crash immediately nor almost immediately for that matter. You can play almost an entire minute of it before it crashes.
The water slides don't play any sound effects and Metal Sonic doesn't have a proper animation for them, but other than that they work as intended. 
Inhaling an air bubble softlocks the game as it locks Metal Sonic in his idle animation and prevents him from moving. Luckily, Metal Sonic can breathe underwater, so the air bubbles can be avoided. 
Just like in Splash Hill Act 2, there are a few spots where the background disappears. 
The level will inevitably crash when you reach the part where you tilt the screen. 
Lost Labyrinth Boss: Trap-Filled Ruins#
This is the only boss act that doesn't crash immediately. That being said, it crashes almost immediately because of the boss. There is also one spot where the background disappears. 
Mad Gear Act 2: Escape the Cog Trap#
Mad Gear Act 2 is the only unused level - besides Splash Hill Act 2 - that doesn't crash at any point.
All the gears have messed up collision, but are somewhat working as intended. The doors that are opened by the large gears close almost immediately. While it is barely possible to get trough the first two doors, getting through the third is impossible. As such, the level can not be completed. Once again, all these objects are missing their sound effects. 
Mad Gear Act 3: Impending Doom#
Just like Splash Hill Act 3, this level is missing its act specific lighting. The wall that's supposed to be chasing you is missing and while that doesn't crash the game, the moving pistons that try to lock you do. 
Mad Gear Boss: Defeat the Real Dr. Eggman#
This level crashes immediately.
Video Showcase#
All the unused levels can be seen here: https://www.youtube.com/watch?v=CvAHcqOimbg
Note: The music in that video is not in the files of Sonic 4 Episode 2 and is added for aesthetic purposes only.