What R-value is 4 inches thick?

What R-value 4 inches thick gives depends entirely on the insulation material because R-value per inch varies widely. Four inches of closed-cell spray foam at R-6.5 to R-7 per inch provides R-26 to R-28, making it one of the highest R-values for 4 inches and also an excellent air and vapor barrier. Four inches of polyisocyanurate rigid foam board at R-6 to R-6.5 per inch provides R-24 to R-26, though its performance drops slightly in very cold weather. Four inches of extruded polystyrene XPS at R-5 per inch provides R-20. Four inches of expanded polystyrene EPS provides R-14.4 to R-16.8 depending on density. Four inches of dense-pack cellulose at R-3.2 to R-3.8 per inch provides R-12.8 to R-15.2. Four inches of fiberglass batts provides about R-12 to R-15 depending on density, with high-density batts giving more. Four inches of mineral wool batts or boards at R-3.3 to R-4.2 per inch provides R-13.2 to R-16.8. So the answer ranges from about R-13 for cheap fiberglass to R-28 for closed-cell spray foam for the same 4 inch thickness, which is why material choice matters so much.