Which of the following is not a net of a cube?

1. Analyse Diagram (1) to see if it forms a cube

Diagram (1) has a row of 3 squares. The squares below and to the right can fold to form the remaining faces without overlapping. It is a valid net.

2. Analyse Diagram (2) to see if it forms a cube

Diagram (2) is a step-like arrangement (often called the 'staircase' net). When folded, the faces meet perfectly to form a cube without overlapping. It is a valid net.

3. Analyse Diagram (3) to see if it forms a cube

Diagram (3) has a central row of 3 squares, with one square above and two squares below. The two squares below are offset such that they will fold to form the bottom and a side face without overlapping the top face. It is a valid net.

4. Analyse Diagram (4) to see if it forms a cube

Diagram (4) has a central row of 3 squares. There is one square above the middle square, and two squares below the middle and right squares. When folded, the two squares below will overlap with each other or leave a gap, failing to form a closed cube.