The density of a block is determined by its mass and volume. The first sketch (Figure 1) shows a rectangular block with a mass of 11g and dimensions of 4 cm x 3 cm x 2 cm. The fourth sketch (Figure 4) shows the same block with a mass of 16g and slightly larger dimensions of 5 cm x 3 cm x 2 cm.

From the given information, we can calculate the densities of the two blocks. Using the formula D = m / V, the density of the first block is calculated to be 2.75 g/cm^{3}, while the density of the fourth block is calculated to be 3.2 g/cm^{3}. Therefore, the density of the block in the fourth sketch (Figure 4) is higher than that of the block in the first sketch (Figure 1).

It is easy to observe that the increase of the block’s mass in the fourth sketch (Figure 4) is not proportionate to the increase of its volume. This leads us to the conclusion that the block must have become denser in the fourth sketch, explaining why the density of the fourth block is greater than that of the first.

In conclusion, the density of the block in the fourth sketch (Figure 4) is 3.2 g/cm^{3}.