To calculate the r-value of a material, we use the formula:

r-value = thickness (in inches) / thermal conductivity (in Btu-in / hr-ft^2-°F)

Given that the material has a thermal conductivity of 2.0 Btu-in / hr-ft^2-°F and is 4 inches thick, we can calculate the r-value as:

r-value = 4 in / 2.0 Btu-in / hr-ft^2-°F = 2 hr-ft^2-°F / Btu

Therefore, the r-value of the material is 2 hr-ft^2-°F / Btu.