Welcome to the forum!
If you use the HEX modifier while receiving the ASCII string each byte in the string/array will contain the numeric value (0-15) represeneted by the ASCII character (instead of the ASCII code FOR the character). Once you have that it's just a matter of taking the first byte, shifiting it 4 steps to the left so it ends up at the four most significant bits and then add the next byte.

/Henrik.