This is a simple two bit to four bit extender. You can select between sign extension or zero extension with the bit S.

S0 and S1 are the extended bits, while A0 and A1 are the original input numbers.

When the bit S is set to 0, the extension is done by setting S0 and S1 to 0. If the S bit is set to 1, the extension bits S0 and S1 depends on the A1 bit, if it is 1, then both S0 and S1 are 1 and if it is 0, then S0 and S1 are 0.

Warning: this schematic uses generic gate symbols. If you want to build the circuit, you can use chips from the 74xx series or from the 4xxx series.