How do I get Xcode 4 to NOT clear the log between runs?
I feel the same way you do about this. That's why I've filed a radar at bugreport.apple.com. Please reference "Problem id: 9153445" when you filed your radar as the this will increases the incident count and making it likelier Apple Xcode team will prioritize a fix.

Since bugreport.apple.com doesn't allow you to search and view another's radar, I've copied the radar I filed at Apple at http://openradar.appspot.com/radar?id=1144401.

Thanks for reminding me that this regression needs people to start filing radars.