Confused about yeast amounts in two of Reinhart's recipes.

I doubt it is a typo, it is just where Peter's baking was at when he wrote the BBA. His epiphany then was that lower temperatures and slower fermentation with less yeast led to more complex flavour. But I think that his techniques then were not optimized for whole grain baking. As you've seen, as he focused more on whole grain he seems to have concluded than when baking whole grain a bit more yeast leads to a more consistent rise and does not significantly detract from the flavour of the final loaf.

My advice is to experiment and figure out what works best for you. If 8g of yeast is working for you but you want to try something different, try the same recipe but scale it back 6g. I would expect the rise to take longer, but it might develop differently in a way you enjoy. Or not! If you've found something you like that is easy to reproduce, there is no shame in sticking with it.

There is no typo, Crobran. These are two very different breads to begin with. Different in composition and in the methods.

And 4 g of instant yeast per 567g of flour is equal to 2% of compressed yeast in light and holey French bread (baguettes, etc.), hardly a brick. 

Peter Reinhart always indicates both times and volumes in his books which help. His 4g of yeast bread dough from WGB is a warm dough at the beginning of fermentation (81F/27C) and it ferments much longer than the 8g of yeast recipe (2 hours at medium warm room temperature vs BBA bread which ferments for only 45 min at cooler room temperature), until the bread dough doubles in volume, and then it doubles in volume again in the pans, and then rises even more in the oven. So, the total rise in volume is 4-6x, hardly a brick.

So, if you pay attention to those details, your 4g of yeast mixed grain (20-25% coarsely milled rye, barley, corn or oats, or coarse wheat meal) water-based bread from BBA will be a beauty to behold. It has no fat in it and its big volume and open crumb are due to high-protein whole wheat flour used as a base, and warm dough temperature and warmer fermentation temperature and proof. It is similar to cracked wheat bread. 

His 8g instant yeast bread from WGB is milk based, 100% whole wheat flour bread rather heavily enriched with sugar and fat. Any whole wheat flour would work. It is enriched with both 3 Tbsp of sugar and 1 Tbsp of butter/oil. It is a colder dough that ferments at rather cool room temperature (21C/70F), the aroma and taste, let alone its look, will be different from the other bread from BBA.