Hi,

**a** = 2...from **d** x 4 = even, and **a** x 4 must be a single-digit result (to avoid a 5-digit end product).

**b** = 1...from **b** x 4 must be a single-digit result (to keep **a** = 2), and 2 is already taken.

**d** = 8...from **a** x 4 = 8.

**c** = 7...from (**c** x 4) + the tens unit (3) from **d** x 4 must end in 1, which is only possible with **c** = 7 or 2; but 2 is already taken.

So, abcd is 2178.