Own region updates do not tend to brick Wiis, it's when you update with a game that's from, in your case E or J locality that you run the risk.
Seeing as you are already at 4.0U then you've got two choices...
1) Update when it asks and it will probably just add a very small change to your existing IOS setup that will allow the game to run.
2) Find out which IOS you are missing and add it manually using wad manager.
I don't recall running a system update for Wiifit although the kids could have done that as they tried it first. I also have a mod chip so am not overly concerned by updates.