I don't see why you shouldn't be able overclock it a few .1's with a stock speed of 3.9 going to 4.2 would only be a 0.3ghz increase however this can bring high temperature increases, I recommend increase it by .1ghz and stress test it for 15 minutes to check the temps until your temps are rising to around 70-80, it is not a great idea to run processors at too higher temperature as it reduces their life span however if you are trying to squeeze more power out of you chip then you should be okay. I would however recommend upgrading to a better cooler in the future to keep those temps down