C-MOS 4 input NOR gate
We're going to make a 4-input NOR gate using two 2-input NOR gates and two 2-input NAND gates. The output of this NOR gate is 1 only when all four inputs are at 0, and it goes to 0 if any of the four inputs is at 1.
After wiring the project, switch power ON and see what happens to the LED. It lights up (output is 1). This is because S1 - S4 are still OFF at this time and the inputs are at 0. Press any of S1 - S4, and the LED goes out. See how the LED blinks ON and OFF as you press/ release the four keys.