I still have a lot of the 5W charging bricks around the house, and I'm wondering since only one side of the cable is different, whether it can be used to charge the iphone 13 models.
Yes. Absolutely.
The bigger 13 pro models have much bigger batteries than the iPhone 4 which that 5 watt charger was initially designed for — but that little charger will work. It'll just take a long time.
Sure 5W works but it is quite slow.
I'd recommend getting a 20W USB-C adapter it costs around $10-20 but it's quite convenient (Anker or Apple are your best bet). Def worth it IMO