The fourth channel is the alpha channel. The EFB stores all four channels, but the EFB itself is not a channel.
EDIT: The article just reminded me that I've missed a detail. Dolphin doesn't use 6-bit alpha channels for everything now, but it does always use 6-bit alpha channels when storing to the EFB. (Maybe the same is true of the RGB channels when the 24-bit option is off? I'm not sure, since haven't worked on the graphics part of Dolphin.)
EDIT: The article just reminded me that I've missed a detail. Dolphin doesn't use 6-bit alpha channels for everything now, but it does always use 6-bit alpha channels when storing to the EFB. (Maybe the same is true of the RGB channels when the 24-bit option is off? I'm not sure, since haven't worked on the graphics part of Dolphin.)