Converting from battery powered to USB

Would I need to use anything like resistors or anything?

I plan on removing the battery pack and soldering a USB cable instead.

The item takes 3 AAA batteries. But I will have 4 of these going into a USB 4 way splitter.

You can connect one silicon power diode cathode to positive of the USB cable. That will loose 0.6 to 0.7 volts supplying your item with 4.3 to 4.4 volts. Using 4 items may over load your USB bus resulting in less than 5 volts on the USB bus. The diode is much better than a resistor, which is more likely to over load your USB bus or under power your items. Possibly you can just make the connection with neither diodes or resistors. Neil