If i purchase another battery do i need to do anything to it before i use it for flight? Like an update or something? And if i do can i still use the old battery without the firmware update

DJI Go 4 will let you know if you need to update the battery.

With the RC turned on and the DJI GO 4 app up and running on your mobile device, plug the new battery into the P4Pro, power it on.  Wait - in less than 20 seconds if you need to update the battery firmware you will get the following message box on your screen "Inconsistent Firmware Found" and you will swipe right to start the battery firmware update process. You will know the battery firmware is being updated when the the two center LEDs (and no others) on the battery light up and start blinking. Updating battery firmware typically takes less than 2 minutes. You will get an "Update Complete" message. If you have more spare batteries, power down the P4Pro, change out the battery with another spare, and repeat the process.  

Note: After updating battery firmware, you should always power down the battery and restart it before flying.

What if I don't have the latest firmware on the phantom/RC? I'm afraid to update because the drone flys flawlessly right now

If the new battery needs an update, DJI GO 4 will let you know, so better test it before the actual flight. If the old battery doesn't need to be updated you just keep using is . In any case the App will tell you .