Plan a circuit to convert a 4 digits binary number to two's complement
A "two's complement" is just a one's complement followed by a increment.  For example:

0101  &nbsp;original
1010  &nbsp;one's complement
1011  &nbsp;plus 1

The orginal was 0101, which is 5, and the result is 1011, which is -5 as expected.  Put another way, it takes 5 increments to make the result 0.