The primary difference between a 3-inch and a 4-inch printer is the print width, with the 3-inch printer (like the ZY310) better suited for smaller labels, such as e-commerce tracking numbers, and featuring dual-mode functionality for labels and bills, while the 4-inch printer (like the ZY909) is designed for larger labels, primarily used in warehousing and logistics. The choice between the two depends on the specific needs and label size requirements of the business.