This is my first 4 bit adder built with nothing more than relays.
The forest of wires on the right are used to set the input values (in the picture Input1 = 0100 (4 in base 10) and Input2 = 0101 (5 in base 10)), and the LEDs on the left are used as output indicators for the SUM (in this picture 1001 (9 in base 10)).