Believe it or not, we just found the fix last night; this is a known issue with GE 4.0 beta. Essentially, in Google Earth, go to the Tools Menu, then select Options. Click the Cache tab, and change the value for Memory Cache size to something larger. Mine was originally set to 80 megs, I upped it to 200 megs and it solved the problem - however, you may be able to get away with a smaller number (or you may need an even larger number).
Give that a shot and let us know how you make out...!
Andy, service@fboweb.com