Two tests are needed to verify the four-element circuit. In the first test, put 0 in inputs A, B and C, and 1 in input D; output E should be 1. In the second test, put 1 in inputs A and C, and 0 in inputs B and D; output E should be 0. If the circuit has four AND gates, three tests would be required, with the following inputs: 0111 (meaning input A is 0 and the others are 1), 1011 and 1110.
Only two circuits in the three-element configuration cannot be verified with one test: the circuit in which element 2 is an AND gate and the others are OR gates, and the circuit in which element 2 is an OR gate and the others are AND gates.