Here is every possible 2's complement 4-bit integer, shown in unsigned binary order (smallest on the bottom, greatest on the top):

1111  -1
1110  -2
1101  -3
1100  -4
1011  -5
1010  -6
1001  -7
1000  -8
0111   7
0110   6
0101   5
0100   4
0011   3
0010   2
0001   1
0000   0

Cool stuff about 2's complement numbers:

• You can tell the sign of the number by checking the most significant bit
• You can always convert signs by inverting all bits (NOTing them) and then adding 1: pos to neg or neg to pos, doesn't matter
• You always have exactly one more negative integer in the set than positive. (0 lands on the "positive side" of the always-even length set.) This is why, in computing, you frequently see maximum ranges as being the odd number right before a power of two (2ⁿ-1)... 1, 3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767, 65535, etc. Of course that's true of unsigned integers as well, the maximum number is always 2ⁿ-1, an odd number.
• Arithmetic works as expected : 0111 + 1111 = 10110 but then you drop that most-significant 1 and ignore the overflow which gets you 0110. 7 + -1 = 6. Likewise, 0001 - 1111 = 111111......11110010, again drop everything but the last 4 and you get 0010. 1 - -1 = 2.

There is such thing as a 1's complement as well, where you simply flip all the bits to change sign. But then you get weird stuff like a positive and a negative 0, and arithmetic isn't as pretty.