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.
IMO, I would take it to about 1.45v just to see what it yields, given you are running stress tests to check for stability. I wouldn't go all that much past 1.4v for constant use as other people have roughly said. Your at a good setting already if you ask me, but if your asking if it is safe to test test the limits, just make sure you realize that voltages above 1.4v is probably not good for your cpu. I have pushed an e5200 voltages to the limits of the motherboard but it's not recommended and doesn't yield the gains that you would think. 1.5v is about where you reach the performance/volt ratio. (this is with the older stepping) Sure you can push it further but it doesn't get much better then that unless you are pushing 1.8v+ with significant cooling methods.