I currently use an IPhone XS on vodafone in the UK. I want to unlock my old iphone 4 which I have not used for years and wipe the contents and the settings so that I can give it to my son who will then use it with a new giffgaff sim

I am UK based and use and use an iPhone XS with a Vodafone SIM. I used the same SIM on my old iPhone 4 many years ago. I want to access my old iPhone 4 so that I can wipe all the settings and data in order that I can give the iphone4 to my son to use with a GiffGaff SIM. What is the best way to do this without messing up anything to do with the data and settings my iPhone XS?