Step 1: Things You Need:
2. a SD card
3. a pc with internet access (of course)
Step 2: Action!
Follow all of the bannerbomb instructions and you will have the home brew channel on your 4.0 Wii!!
If this doesn't work for you, I'm sorry but you'll have to wait for a new exploit.
What can you do with the homebrew channel you ask?
Play games, play video and audio, and various other activities!
More information can be found here: http://www.wiibrew.org/wiki/Main_Page
Do not pay for homebrew or the homebrew channel
If you have a wifi enabled Wii I suggest: http://www.wiibrew.org/wiki/Homebrew_Browser