Sign Extension
When you do sign extension, you take the higher bit (sign digit) from small-bit-number and fill the rest of the most significant bits in the larger-bit-number with that sign digit.
Example:
Sign extension
4-bit to 8-bit
1000 -> 11111000
0101 -> 00000101
Harris D. M., Harris S. L. - Digital Design and Computer Architecture, 2nd Edition - 2012
1.4.6 Signed Binary Numbers - 18 page