Variables:
X1::[1..4]
X2::[1..4]
X3::[1..4]
X4::[1..4]
Constraints:
X1 != X2
X1 != X3
X1 != X4
X2 != X3
X2 != X4
X3 != X4
X1 != X2 + 1
X1 != X3 + 2
X1 != X4 + 3
X2 != X3 + 1
X2 != X4 + 2
X3 != X4 + 1
X1 != X2 - 1
X1 != X3 - 2
X1 != X4 - 3
X2 != X3 - 1
X2 != X4 - 2
X3 != X4 - 1