last year after week 4 there were 27 undefeated teams.
'11- 22
'10- 25
'09- 17
'08- 27(best comparison of all, due to the fact that it was a similar calendar to this year.)

So, 27 seems to be about normal.
'13 undefeateds after week 3- 35
'12 after week 3- 31
'11- 33
'10- 34
'09- 28
'08- 39

last 5 years- average after week 3- 33 this year 35(makes sense with the extra teams for instance)
last 5 years- average after week 4- 23.6

So, this is the week that we normally see a lot of undefeated teams lose- on average almost 10 per year. Last 5 years by end of next week, we're down to 17.4 undefeated teams. So, last 5 years, we've gone from week 3 33 teams undefeated down to week 5 17.4 teams undefeated(losing 15.6). That's like 47% of the undefeated teams.