I don't know what it is off the top of my head, but I would be willing to guess that it's somewhere between 5.25 and 5.50, perhaps higher. Another interesting study is to look at Mark Prior's ERA based off of pitch counts. When you are looking at ERA after the first four runs, you're adding a second very very high variable: fatigue. Personally, the way I would have adjusted for this is by looking at two groups, a weighted ERA based on the pitch counts where he was giving up the runs, versus the same pitch counts weighted averaged to when he had only given up one or no runs up to the point. Essentially giving meaning to this by factoring out fatigue, obviously the control group would be biased towards his good starts. If there is little to no difference between the two (which I suspect) then we can certainly conclude that Mark Prior does not self-destruct.