The 4.00 - 6" tire is common on 4 and 5 cubic foot wheelbarrows. Our 3.50 / 2.50 - 8" is the equivalent tire to a 4.00-6", so you will receive the 3.50/2.50-8" tire if you order a 4.00-6". Our 3.50 / 2.50 - 8" tire has the same 13" outside diameter, but due to a large rim, is a low profile version of the 4.00 - 6".