In Figure E6-3, which is the schematic symbol for a NOR gate?

A NOR gate combines an OR gate with a NOT gate (inverter). The standard symbol for an OR gate has a curved input side and a pointed shield-like output side. To make it a NOR gate, a small circle (inversion bubble) is added to the pointed output. In Figure E6-3, symbol 4 shows this exact shape: a curved input side, a pointed output, and a small circle. Symbol 3 represents a regular OR gate (no circle), while symbols 1 and 2 represent AND and NAND gates, respectively.