To calculate density, you use the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
Given the mass is 12 grams and the volume is 4 mL, the density would be calculated as follows:
\[ \text{Density} = \frac{12 \text{ grams}}{4 \text{ mL}} = 3 \text{ g/mL} \]
The correct unit of measure for density in this case is grams per milliliter (g/mL).