Hmmm.... Interesting....
Regulars on this board will know I'm one of the very few that actually go on record and agree with John Lucas' predictions. But wow John, that's quite a call.
Now Japan is an obsessive type nation, and immense first day sales have been seen, most recently with the PSP Slim/FFVII/MGSOP launches. So that's supports you.
Wii Fit will appeal to a MUCH larger base than the Wii alone would, so that definately supports you.
However...
Nintendo hasn't been able to produce sufficient Wii or DS to meet demands all year, so for them to pull 4m+ Wiiboards out of their arse for one weeks sales seems incredulous. You say they won't leave money on the table, but I submit, they have all year.
Nintendo is nortoriously conservative, while you may predict that amount of success, I guarantee Nintendo does not.
Nintendo is also proven to be fairly lousy at marketing. Now Wii Fit, like the Wii before it will create it's own marketing without Nintendo's flimsy marketing department's help, but, that takes some time.
I've predicted Wii Health Pack (as it was known at the time) would sell like nothing ever seen before it on this forum long ago, but since then Nintendo revealed Wii Fit and the Wii Board and I was underwhelmed by it's potential. Having said that, I think it is directed more at the Japanese than American market and will have greater success there than here (but it will still be big here). I still hold to that claim, but...
I'll go on record here as saying John Lucas is wrong. Wii Fit in Japan will NOT meet Halo 3's WW first week sales. But you will all be amazed by how close he comes.