The solid below is made up of 1 cm cubes. How many more 1-cm cubes must be added to the solid to make it a cuboid measuring 4 cm by 3 cm by 4 cm?

1. Calculate the total number of 1-cm cubes required to form the target cuboid.

$\text{Total cubes} = 4 \times 3 \times 4 = 48$

2. Find the number of cubes to be added by subtracting the existing cubes from the total required.

$\text{Cubes to add} = 48 - 21 = 27$