74LS21 DUAL 4-INPUT NAND GATES
The 7421 IC is composed of two independent Four-Input AND gates.
The truth table for the 4-input of the AND is identical to that of the 2-Input variant. Naming the logical inputs and outputs as zeros and ones allows for easy calculation of the function performed.
Let inputs be marked as A, B, C, and D. The output will be called X.
Then, A * B * C * D = X. For example, if 2 of the inputs were logical one and two of them were logical zero, then 0*0*1*1 = 0, therefore the output of the gate would be zero.
By a simpler method, simply consider the fact that the gate will only be output logical one if and only if ALL inputs are logical one (HIGH).