Solution:First letter : V -2 = T , T - 2 = R, R - 2 = P , P -2 = N
2nd letter : Z - 2 = X, X - 2 = V, V - 2 = T , T - 2 = R
3rd letter : L - 2 = J, J - 2 = H , H - 2 = F , F - 2 = D
4th letter : 15 + 3 = 18 , 18 + 3 = 21 , 21 + 3 = 24 , 24 +3 = 27
So, ? = NRD 27