http://www.hardballtimes.com/how-good-is-your-4-starter/

How Good Is Your #4 Starter

To address the issue I raised at the outset, we can use these averages to
come up with rough dividing lines between rotation spots. Armed with this
data, you can take any pitcher’s ERA and eyeball where they would fit in to
the average team’s starting corps. For instance, in the table below, if a
pitcher is between 3.87 and 4.36, he is, on average, a #2 starter.

Spot    MLB     AL      NL
#1/#2   3.87    3.97    3.78
#2/#3   4.36    4.41    4.31
#3/#4   4.84    4.84    4.84
#4/#5   5.67    5.66    5.68