I tried to use this 4.0 downgrade method, but it resulted in a semi-bricked Wii.
Wii info:
Korean Wii with 4.0u Serialnumber: LKM10572987, Copyright 2008
Bought it in Hong Kong last month
HBC + bootmii installed (IOS)
How did I brick it:
I was at step 7 of this guide...