Alas, I don't have an entry yet for Episode 4 -- we were running late last Sunday and had only sat down to dinner at 9pm, so it's hard to eat and make notes at the same time. Was going to catch the NatGeo rerun on Monday, but I was racing to finish my WOTF story.
The episode was about how damn big the universe is. The story of Herschel, who realized that looking into deep space was looking back in time. Hundreds and thousands of years. It would be a while before we understood it was millions and billions of years. Also, light and gravity and black holes.
I'll catch up with Episode 4 sometime and update this entry -- meanwhile, here's a placeholder.