4 Digit Number

$\begin{array}{ccccc}A& B & C & D \\ \times &&&9 \\ \hline D & C & B & A \\ \hline \end{array}$

Here $A = 1, B=0 , C = 8$ and $D = 9$

$\begin{array}{ccccc}1& 0 & 8 & 9 \\ \times &&&9 \\ \hline 9 & 8 & 0 & 1 \\ \hline \end{array}$

Hence $A \cdot B \cdot C\cdot D = \boxed{0}$

We can say that the highest result of DCBA is 9999. So the highest result of ABCD is 1111 (9999/9=1111). We see that ABCD is not equals to 1111( because if ABCD is 1111 then DCBA is 1111).ABCD is also can't be 1110. So ABCD is between 1000 to 1109. We see that either B or C is 0. So A * B * C * D = 0.