The equation for density:
#rho = m/V#
Plug in known values, if your answer is in units of #(kg)/(m^3)#:
#rho = (4kg)/(20m^3) = 0.2(kg)/(m^3)#
A more conventional unit for density is #g/(cm^3)#. The conversion to this is:
#0.2(kg)/(m^3)((10^3g)/(1kg))((1m^3)/(100^3cm^3)) = 2 xx 10^(-4) g/(cm^3)#