Is there a difference in light output between a 4 watt 12 volt incandescent lamp and a 4 watt 120 volt incandescent lamp?  I'm curious because the 12 volt automotive lamp used in the trunk of my car seems to be about the same brightness as a 4 watt C7 120 volt night light bulb, at about 15 lumens.  Does voltage affect light output if the two lamps are rated at the same wattage (i.e. 4 watts)?

The 12V filament is thicker for the same power and can be made to run hotter. But....

 - 12V system is way more sensitive to voltage drop. The trunk light performance is not that critical, so it can be wired with thin wire that can have significant voltage drop. Aging lamp holder and switch contacts may oxidize and add to the voltage drop

 - The light fixture in many cars is very inefficiet - It might be just letting light go into the wall cavity from the back, or be made of not highly reflective materials inside. Most "120V" lighting is much better

 - The lamp itself, most likely a "double ended" design may be made to not run very hot, in order to try to prevent problems like filament sagging, vibration susceptibility and such

 - The conditions under which you use a 4W lamp at home vs. lamp in the car trunk are very different. If you compare the light output "from memory" (and not side by side), you introduce a measurement error which may be on the same order as all of the above factors

If all other design factors are the same, at such low levels as 4W the 12V could run hotter, so have higher efficacy, the reason is the thin 120V filament needs more margin for mechanical rigidity.
But automotive lamps have to endure a lot of vibrations, so a 12V 4W auto lamp has to operate at lower temperature so have lower light output than would a 12V 4W home use have.
So it became a coincidence the 120V 4W home lamp has the same output as the 4W 12V automotive.

120V automotive wont be feasible at all, with 24V (large truck or bus lamps) the power rating has to be already about 20% higher or light output 20% lower than a 12V equivalent for the same function. Well, with most lamps the wattages are kept same and the lower output is tolerated, for the front lights however the wattage is increased (e.g. 24V are 75/65W instead of 65/55W for 12V H4...)...