First make a sourdough starter. Crumble yeast into a large bowl. Whisk in 1 quart of warm water (110º) and 2 tablespoons sugar, until dissolved. Gradually whisk in 4 cups of flour, continuing until all lumps are gone. Cover with a dish towel, and let sit for 24 hours at room temperature.
After 24 hours; stir well and let stand another 24 hours. It will be thin, light-colored sourdough which is then ready for use.
In a large bowl; stir together the rye flour, 4 cups white flour, salt, and sugar. Mix in the sourdough starter using a wooden spoon, add in 2 cups of warm water; mix well. Transfer to a heavy duty mixer with dough hook; mix for 2 minutes, until it can't handle the heavy dough. Turn the dough out onto a floured surface and use your hands to continue mixing.
Knead the dough, adding a few tablespoons of water at a time if the dough is too stiff. Fold the dough over, pull it apart, what ever you need to do to get it kneaded up good. Total kneading time should be 15-20 minutes, until smooth and elastic. Place dough in a large bowl; cover and let rise, until double in size; about 1-2 hours.
When the dough has risen, scrape it out of the bowl and onto a floured surface. Knead another 5 minutes. This is important to activate the gluten. Shape into 1-2 long loaves.
Place on baking sheets, let rise another hour or until finger leaves an impression when you poke the bread gently.
Preheat oven to 425º. Bake for about 45 minutes for 2 loaves ands 1 1/2 hours for one big loaf.
Don't worry if the crust is dark. The bread will be delicious and so will the crust. Cool completely before cutting.