Looking at the 4th net, we see it has what seems to be 4 squares in a line. And two of them are each one united into another square.
Then, when we join the left side of the left square and the right side of the right square, we obtain a 3D object with 4 square faces, and 2 empty faces also in the form of a square.
After that, we can fold down the upper square face, and fold up the other one.
We will get the following 3D object, that is a cube:
Therefore, the 4th net makes a cube.
