To downgrade 4.2 to 4.1, downgrade IOS60 to v6174. This is VITAL, otherwise you will get a nasty brick only BootMii as boot2 can fix!
After that, you should be able to install the System Menu 4.1 WAD with no issues.
sorry for not know for sure but is this what you are talking about
Downgrading your Wii
Run WAD Manager 1.5 from The Homebrew Channel.
When it opens choose IOS249. (it is probably already set to that)
Press A to leave NAND emulator disabled.
Press A to open from SD slot.
*Really important stuff here! Do not skip or alter steps in any way!*
Install IOS60-64-v6174 [TB+ES_diV+NAND].wad
Install System Menu-NUS-v449.wad [NTSC-J = v448; PAL = v450]