Dave, thanks for posting a photo of the soapstone. The veins are little creamier than I like, but I think it a perfectly good stone.
4 days is, imo, kind of advanced pizzamaking. With that length of time, you're really in a potentially sketchy territory when it comes to gluten degradation/enzyme activity. You also could be producing too much alcohol and adversely affecting the flavor of the dough. I would save the extended fermentation until you've completely mastered the oven spring issue, and, for now, shoot for a 2 day dough. Ideally, you want to use just enough yeast so the dough doubles in the fridge and, by the time you stretch it out, it will have tripled. I would stick with a half teaspoon and shoot for 2 days. It the volume increases too quickly, use it after a day and dial it back next week.
And, although I'm giving you conflicting advice here, I'm not a big fan of using warm water with yeast. I strongly believe that the only thing yeast has to do prior to combining with the flour is dissolve in the water. Room temp water and a minute or two of time will achieve this. Scale out the water first, sprinkle on the yeast, give it a stir, and, by the time you have scaled out the dry ingredients in another bowl, the yeast will be fully dissolved. Yeast activity generates heat, so the warmer the dough is going into the fridge, the more yeast activity you'll have/the longer it will take to chill, which, in turn will generate even more yeast activity. Ideally, you want the dough to be pretty close to room temp going into the fridge so the yeast doesn't go crazy at the beginning of the process. I know some people use ice water, but, imo, it's just so much easier to start off with room temp water.
As Gtsum2 mentioned, unless you had darker areas that you didn't take a picture of, the bottom of this last pie was pretty much perfect, imo. That's how the bottom of the crust is supposed to look. If you want a little more even coloring (both on the bottom and the rim), try adding a little sugar. 1% is a good jumping off point.
Isolating the bottom of the oven with foil is a good idea, although I'm a bit stumped as to what stone temp to shoot for, as the outside of the stone will get very hot quickly while the core will be quite a bit cooler. I would try preheating the stone for 40 minutes, checking the temp, and if the top of the stone is close to 575, launching the pie and then cranking the heat so it collects under the foil and browns the top. If it's a lot higher than 575, turn the oven off until it cools to 575. I would also go with the same 4" vertical space with the foil that you were using with the broiler.