I was playing this today, definitely 4:3 only....
Mario Party 8 is a noteworthy arrival on Wii because it marks the first Nintendo-published project that does not run in 16:9 widescreen mode. To our knowledge, this hasn't happened since the days of GameCube and quite possibly with the previous Mario Party outing. To its credit though, the game's title screen and main menu both run in full widescreen, but as soon as any sort of gameplay begins, everything transpires in 4:3 with superimposed bars.
EDIT you can kind of bodge it by selecting 4:3 from within the wii main menu before starting the game, it will then stretch it over the whole screen, just seen this on youtube.