The “4. In this case, the tire is 4. The next digit represents the wheel size the tire will fit. This tire is meant for trailer wheels that are 8 inches in diameter. As a general rule, you want replacement tires that are within 3 percent of the diameter (height) measurement of the tires originally on your vehicle.The overall diameter of a wheel and tyre fitted to a vehicle must not be more than 50 mm (4WDs) or 15 mm (other vehicles) larger than the largest tyre size listed on the tyre placard and not more than 15 mm less than the smallest tyre size listed on the placard on all vehicles.This indicates the tyre’s width in inches. In this case, the tyre has a width of 4. This represents the tyre’s load index, indicating its maximum load capacity. A higher number indicates a higher load capacity. This denotes the tyre’s rim diameter in inches. The numbers 4.