If it stays cool enough and you dont have to touch the voltage to get it to 3.9, I guess it would be safe. And turbo boost will push it up to 3.9ghz when temps allow at stock anyways so I doubt you would gain all that much performance at only 3.9ghz. Personally I would never try overcloking on the stock intel cooler. What would be your budget for a cooler? Lots of options out there.