So suppose it is ABCD
C is twice D and A is twice C. So A is 4 times D, so the number is either like this 4B21 or like this 8B42
The largest B can be is 9 and 4921 would only have a sum of digits 16. So the answer is 8B42 and B has to be 5 to make the sum of digits 19.
8542 Edwin