The density is the amount of mass per unit of volume of any material. You can find the density by using the formula:
[tex]d = \frac{m}{v} [/tex]
where d is the density, m is the mass and v is the volume
you have the mass and the volume so you just have to replace the variables in the formula with the values they are giving you, like this:
[tex]d = \frac{m}{v} = \frac{48g}{4ml} = 12 \frac{g}{ml} [/tex]
The density of lead is 12g/mL