These averages include sporting events like the Olympics and Super Bowl which typically have noticeable effects on the averages. Ideally I'd love to get averages of just entertainment programming, but this is all I could find for now.
*- The number I saw for CBS came before finals were available on the last day of the season, and it wasn't certain whether it'd end up being a 3.9 or 4.0. So I went with 3.95, which is likely even more accurate than the other two numbers would be.
*- This was the year when the WGA strike shortened the seasons of almost all scripted shows. Aside from Fox, which benefited from the Super Bowl and the still full-strength American Idol, it was a rough year for the networks.