Either way will work, but updating officially to 4.0 before installing any cIOS will just limit the number of ways you can install cIOS on your Wii if you want to down the line for a usb loader or something. That isn't really much of a problem now though as there are like 4 different methods now to get cIOS installed (installing IOS16-64-v257-prod.wad, downgrading and installing IOS35-64-v1040.wad, cboot2, and even more recently, the Trucha Bug Restorer) and updating to 4.0 officially will only prevent one of them from working (IOS16 install).