Okay kiddo, so you know how we live next to a big lake or ocean, right? Well sometimes, there are places where there's an extra bit of land that sticks out into the water. This extra bit is called a "shore".
Now, when we talk about a "fourth shore", we're talking about a special kind of place where there isn't just one, two, or even three of these shores, but there are four of them! This happens when there is a piece of land that sticks out into the water and splits it into four parts.
It's kind of like if you took a piece of paper and drew lines on it so that it was divided into four rectangles. The part of the paper where the lines cross each other is like the fourth shore. It's where all four parts of the paper touch each other, just like how all four parts of the water touch each other at the fourth shore.
Not many places have a fourth shore, so if you ever get to see one, consider yourself lucky!