2's Complement 4 Bit adder. (Multisim)
I have not analysed the circuit in any great detail, but unless I'm reading your circuit diagram wrong, all of your switches S1 - S8 are wired the wrong way round. In the position shown they all connect the 10V Vcc directly to ground, whilst the resistors R1 - R8 are open circuit leaving the inputs of the gates to which they are connected floating in an indeterminate state. Surely each of the the 1K resistors should be connected to the moving contact of their respective switches and Vcc and ground connected to the two fixed contacts.