Adapting 3xAA christmas light to USB power supply

Tired of charging batteries from time to time, I decided to power supply my christmas lights with a USB adapter.

Since these light uses 3 batteries AA, which is 3 x 1.5V = 4.5V

to adapt the 5V from the USB we need 43 ohm, aprox, so we use 50 ohms resistor.

I tested with the 5V output of my Arduino Atmel, and it work, it also works with 5V, it is brighter, but I am afraid of damaging the lights.