The exploit dubbed as BannerBomb created by Comex is the only public way to load homebrew applications on a System Menu 4.0 Wii. The exploit was released on the 5th April 2009 through the IRC Channel #Wiidev. The exploit works by using a malformed banner to crash the wii loading the boot.dol placed in the root of the SD card. Currently this exploit does not allow the Homebrew channel to be installed due to the exploit that was used being blocked in 4.0. However, it can still be installed through BannerBomb on System Menu version 3.X.

http://wiibrew.org/wiki/BaNNeRBoMB