It all depends. You're very lucky to be hitting that. I'd say go to 4.2 run prime for 1 hour. If it doesn't fail you should be good. However, do try your best to find 5-7 hours to run prime95 just to make sure your system is stable. If it fails, increase to 1.4. If you can't hit 4.2 with 1.4 then you should just keep it where you are now. 4 GHz is good with that cheap anways.