After finding the already tricky location for World 3, I'm sure you're frustrated once again at finding World 4, which, as you've likely looked up is called the Capital Cashino.
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As you've probably noticed, you'll unlock several shortcuts throughout the hub word, Hivory Towers, to help you get around after beating levels. After beating World 3, you'll find another such shortcut with ease by following the grapple points that you see below:
Follow these up and to the right, and you'll find an area that leads back to the main pond (it should be populated by three goons that you should take out). This works as a good shortcut so you don't have to go underwater every time you need to get back to World 3.
But, if you have a quick left right after dropping down to this area, you'll see a big entryway that, yes, wasn't there before:
Tada! Now don't lose too much money gambling in there.