You don't need 4.0, it's not worth updating
Yeah you can store a bunch of games on an SD card, big whoop, whenever you launch a game from there it quickly copies it to the NAND so it still runs them from the Wii's internal memory and copying a game to it EVERY TIME you want to play it can't have a positive effect on the system's flash memory.
Just stick to 3.2, it's the safest and best version, get yourself USB Loader, run games from a hard drive/USB flash drive and have fun.
Sure you can updated safely to 4.0 and crap IF you already have hacks and HBC installed, as far as I know 4.0 isn't fully cracked so downgrading from it isn't as easy if it all possible.
I wouldn't recommend updating, it's not worth the risk even if that risk is minimal IMO.