How to convert from 2s complement values to binary?
This is not an answer, per se.

It is an exploration of how the results of 7 - 4 = -2 and  4 - 7 = -4 could happen.

 7  0000 0111               0000 0111    7         0000 0111    7
-8  1111 1000             + 1111 1100   -4       + 1111 1011    4 inverted
                          ------------           ------------
 4  0000 0100               0000 0011    3         0000 0010    2
-5  1111 1011       

 2  0000 0010               0000 0100    4         0000 0100    4
-2  1111 1110             + 1111 1001   -7       + 1111 1000    7 inverted
                          ------------           ------------
-4  1111 1100               1111 1101   -3         1111 1100   -4