So, I did a bit of digging around in the 510 prototype of Sonic CD, and found 2 additional maps. There's reference to a third one, but the data seems to have been removed (it's just clone of special stage 1's graphics with special stage 3's map).
First, here's a render of the known map (special stage 2 prototype):
Next, here's map #3 (special stage 3 prototype):
And, finally, map #4 (special stage 4 prototype):
And here are some videos of them in-game:
First, here's a render of the known map (special stage 2 prototype):
![f3IoW3q.png](https://i.imgur.com/f3IoW3q.png)
![isoUAqZ.png](https://i.imgur.com/isoUAqZ.png)
![5lzkh1D.png](https://i.imgur.com/5lzkh1D.png)
Next, here's map #3 (special stage 3 prototype):
![0o725JI.png](https://i.imgur.com/0o725JI.png)
![RN1UPKU.png](https://i.imgur.com/RN1UPKU.png)
![6NvApla.png](https://i.imgur.com/6NvApla.png)
And, finally, map #4 (special stage 4 prototype):
![zM8ZvFw.png](https://i.imgur.com/zM8ZvFw.png)
![d20rcL3.png](https://i.imgur.com/d20rcL3.png)
![FxGkPY5.png](https://i.imgur.com/FxGkPY5.png)
And here are some videos of them in-game:
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