What is the difference between a mazda 3 and a mazda 4?

Mazda 3 exists. Mazda 4 doesn't. Did you mean Mazda 5?

The difference between the Mazda 3 and the Mazda 4 is that the Mazda 3 is a compact car manufactered by Mazda. It was introduced in 2004. The biggest difference between the two, is that the Mazda 4, doesn't exist.