You don't have to downgrade your System Menu
Do not try this if you have one of the new LU64 Wiis!
Download
this pack
Unrar the pack.
Rename "IOS35downgrade.dol" to "boot.dol" and copy it to your SD card under apps/DVDx
Rename "WadManager1.4.dol" to "boot.dol" and copy it to your SD card under apps/WADManager
Copy the "wad" folder to the root of your SD card
Put the SD card in your Wii, and start the Homebrew Channel..
Launch DVDx and choose "downgrade IOS35"
After that is done, exit back to the Homebrew Channel
Launch WADManager
Choose all the default settings and install "IOS35.wad"
Exit to the Homebrew Channel and and launch WADManager again.
When it asks you which IOS to use, choose IOS35
Install CIOSv7.wad
If you want, you can update to CIOS Rev 10:
http://teknoconsolas.es/blog/waninkoko/custom-ios36-rev-10
Now you should be able to install wads on 4.0