Look up "Modulo" in mathematics.

You are trying to build a counter that has exactly 12 states. Those 12 unique states can be named anything, including 15,14,12,...,5,4. When you build a counter out of "state elements" (flip-flops), you can arrange them in many ways. You should strive to arrange them so that the numerical value of the states as defined by the binary pattern of the individual flip-flops matches the numerical value such as 15=1111, 14=1110, ..., 0101=5, 0100=4.

With this in mind, you should be able to create a "present state - next state" table.

Question for you: How many flip-flops does it take to represent 12 unique states?