Q:

Override One Lamp in a Four-Lamp Circuit Using Transistor? 

simulate this circuit – Schematic created using CircuitLab
I have a circuit of 4 light bulbs, with single switch on the positive terminal that will turn all the 4 bulbs at once.  EDIT: Light bulbs circuit is parallel design.  source is 12V DC and all bulbs are running 12V DC. 
I want to be able to still have the original switch to turn on all 4 bulbs, with a new switch that overrides bulb #3 to keep it turning on.  Essentially, when bulb #3 is switched on, the original switch ignores bulb #3. 
To solve this I go with installing a diode at the positive terminal of the bulb #3.  When original switch is on, electricity will go thru that diode and then turns the bulb on; when new switch is turned on, electricity will go to bulb #3 thru new line but will not go to the original circuit because of the new diode. 
A friend said this might be able to be done with a transistor but he couldn't figure out the solution.  Is that true we can use transistor for this?  it's for automotive brake lights. 

A:

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 – 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.