Yes, it took a few days for me to actually get to the day 4 lesson, but here I am, finally done! On Day 4, I experimented with ISO. I was familiar with film speeds from my 35mm days, but like the manual settings, somehow my DSLR made me forget much of what I used to know!
I went out into the backyard on a bright sunny day and shot a few close-ups of a cactus. I can see how the image gets grainier from ISO 100 to ISO 1600. I'll include 100, 400 and 1600 as examples. Notice the transition from sharp, clear image to slightly fuzzy. Seeing the evidence so clearly, and now knowing how to actually change the ISO on my camera, I feel like I'll use this feature more often!