The regulations for Tennessee motor vehicle lighting regulations can be found here, under Title 55 Motor and other vehicles, Chapter 9 Equipment – Lighting Regulations.
http://www.michie.com/tennessee/lpext.dll?f=templates&fn=main-h.htm&cp=tncodeI don’t find anything in the code making all four headlamps operating on low beams illegal, but it might be an invitation to get pulled over by a cop who thinks you’re running high beams, or that thinks you have your high beams on and they’re too dim, and you would no doubt be flashed by other motorists that just count the number of illuminated headlights.
The technical problem in accomplishing this is that 4 headlight systems don’t work like two headlight systems. In a two headlight system each bulb contains a low and high beam filament. When the low beam filament is on, the high beam filament is off. When the high beam filament is on, the low beam goes out.
In a four headlight system, the high beam lamps produce all of the high beam light output (35,000 candlepower in a non-halogen bulb).
http://www.napaonline.com/Search/Detail.aspx?A=LMA5001_0278278328&An=599001+101960+50030+2030093It is true the low beam bulbs have two filaments, but the brighter filament is what produces the low beam output (11,000 candlepower in a non-halogen bulb). The low beam lamps do not produce high beam output when switched to high beams.
http://www.napaonline.com/Search/Detail.aspx?A=LMA4000_0278280245&An=599001+101960+50030+2030093On a four headlight system, when the high beam circuit is switched on, the brighter filament in the low beam lamps goes out just like in a two headlamp system. But rather than have the low beam lamps go dark, a dimmer filament is illuminated and acts like a marker light. This is why “headlight brighteners” work. When the high beams are switched on, these devices keep the low beams at full power, providing both distance and close up lighting on both 2 and 4 headlight systems.
http://www.ronfrancis.com/prodinfo.asp?number=BJ-2Replacing the high beam headlights in a four lamp system with two more dual filament low beam lamps, will give you four low beam lamps. There will no longer be high beam light output available. When the high beam circuit is switched on, all four bulbs will switch to the dim filaments providing four nicely glowing marker lights.
