Sign Magnitude, Two's compliment binary representations

The following table shows the difference between Sign Magnitude and Two's compliment

As an additional note, two's compliment involves inverting and adding 1.
So if we look at the table below for 0100 which is 4, the two's compliment representation of -4 would be 1100

How do we get there, well we first invert from 0100 to 1011 and then add 1 to give 1100

To go the other way;
If we have -3 which is 1101 we invert to give 0010 and add 1 to give 0011 which is of course 3

https://en.wikipedia.org/wiki/Signed_number_representations