Override One Lamp in a Four-Lamp Circuit Using Transistor?
Given your control signal is just another switch, your diode approach in the question should work fine.

If the control signals came from two different supplies you would need a second diode in series with the override switch. However as both are fed from the same supply, this is not required.



An alternative solution would be an SPDT relay:







simulate this circuit &ndash; Schematic created using CircuitLab

When you turn the relay on, it will turn on lamp 4 regardless of the state of the switch. When the relay is off the lamp goes back to being controlled by the switch.