StaryStory

@ZiggWheelsManning like I said previously, Ouranos Island used be different in the original game, I can't remember it very well, but I remember it being a lot like Kronos Island, but harder and with some new enemies. The Island's Boss (Supreme) used to be some sort of a reskin of the first boss (Giganto) originally. In the Update, they changed the island to be more original, including Tails, Amy and Knuckles with playable sequences and the Kocos challenges. Supreme was also changed to be a more original boss (with The End attached to it, this latter was a separate boss)

But does a 5th boss come after it?  

@ZiggWheelsManning yes, it's more that "The End" became a phase for it, so at the same time it's the 5th boss, it's also another phase of the same 4th boss. In the original The End was 100% a separate boss, with a different gameplay mechanic and all.