DOP-IOS Mod v8 is easiest. Stay away from cIOS Downgrader, it'll brick your wii if used on any boot2v4 system ("LU64+", or anything with official 4.2).
There really is no reason to downgrade. If you just want to use old HBCs or something, there's a preloader/startpatch hack to remove the anti-homebrew protection in 4.2.