Where to put the unused states in synchronous counter that counts Even numbers?
If you perform a logic minimization analysis of the four state variables A, B, C, and D and take don't cares into consideration in the Karnough map groupings you will find that the D term minimizes away as the input to any of the state transfer equations. Also you will find that the logic equation for the drive of the Q(D) flop flop becomes:

Q(D) := 0