Modify LED strip from 4 AAA battery power to 12V DC
Two options. Replace the resistor, using ohms law to calculate it, or bring the 12v down to 5 or 6v. 12v to 5v adapters are everywhere. A dollar store car usb adapter does that nicely, and it's easier to get than an appropriate resistor.

To replace the resistor, measure the voltage across it with fresh batteries. Then using ohms law, I = V / R, you have all three numbers you need, the voltage dropped by the resistor ohmage and the current through it. You'll want to keep the current the same, so adjust for the voltage difference and increase r until you get the same current, R = V / I. Make sure the resistor wattage is correct using P = V*I