For set A B C, you can enumerate the powerset in binary:

111 ABC
110 AB
101 A C
100 A
011  BC
010  B
001   C
000

For BCD, it would be thus:

111 BCD
110 BC
101 B C
100 B
011  CD
010  C
001   D
000

Instead, we want to offset the values in BCD and leave an empty slot for A. We'll end up with this:

0111 BCD
0110 BC
0101 B C
0100 B
0011  CD
0010  C
0001   D
0000