low voltage LED lighting -- OOOPS wrong place

I've found some LED light fixtures at Harbor Freight that my wife likes. They provide plenty of light where we need it and look like they belong there. They operate on 6 AA batteries but are actually 2 banks of three cells to provide 4.5 volts. I've installed 2 of these in one area and rewired them to connect to a step down adjustable power supply module that reduces the "house" 12volts to the required 4.5 volts. Problem is --- I only ordered one voltage reducer and now the wife wants more lights. On the other side of the room. I'm thinking a resistor could be used to drop the voltage. BUT what value of resistor? I'm assuming the step down adjustable power supply module that I used will keep the output voltage at the 4.5 volts even if the input varies from 12 volts to 14.2 volts. I figure that if a resistor is used, that the supply voltage should be considered as 14.2 when figuring he resistor value to obtain the desired 4.5 volts. But would that then make the lights too dim when on batteries only?