To determine the number of small cubes required to complete the main cube, we first need to understand the structure.

The given figure represents a 3D structure composed of small cubes. To solve this, let's break it down:

1. Determine the size of the main cube:

   • The main cube is $4 \times 4 \times 4$ in dimensions, so the total number of cubes in a complete structure would be $4^3 = 64$ small cubes.

2. Count the number of cubes present in the current structure:

   • We can count the number of visible cubes layer by layer from bottom to top:
     • Bottom layer: 9 cubes
     • Second layer: 6 cubes
     • Third layer: 4 cubes
     • Top layer: 1 cube

   Adding these up: $9 + 6 + 4 + 1 = 20$ cubes are currently present.

3. Calculate the missing cubes:

   • The difference between the total cubes in a complete structure and the current structure gives us the number of missing cubes: $64 - 20 = 44$

So, 44 small cubes are needed to complete the main cube.

Would you like any further details or explanations?

Here are some related questions you might find interesting:

1. What is the formula for calculating the volume of a cube?
2. How many cubes are needed to build a $5 \times 5 \times 5$ cube?
3. How can we calculate the surface area of the current incomplete structure?
4. If each small cube has a side length of 2 cm, what would be the total surface area of the completed cube?
5. How does the number of visible faces change as layers are added?

Tip: When counting cubes in a 3D structure, it's helpful to break the structure down into layers to ensure accuracy.