I think you can do it... I am at this very moment overclocking my CPU (same as yours) and I am at 3876 Mhz, very, very stable, just like the stocked 3400 Mhz. I am running Prime95 at the moment and no issue yet. I set the Bus to 228 to get the 3876 and the HT to 2051 Mhz. Memorys is at 607.9 with 8,8,8,20 (I am using CPU-Z).
I do believe I can bring it to 3995 safely, but, to me, there's no point in that.
Here you are my specs:
AMD Athlon ii x2 270 (processor)
ASUS M5A88V-EVO (motherboard)
8gb Ram Kingstom 1333 9-9-9-24
650 watt ANTEC EARTHWATT (power supply)
Onboard HD4250 384 Mbs (graphics card)
The PC is really fast and powerful even running at 3400 with Microsoft flight simulator, PKR (poker game), Free Video Coverter (that stresses the PC to its limits), Free AUdio Converter, FreeRapid downloader, and a lot of other programs... so, I don't believe I'll go any further in the overclocking. I am really happy with 3876 and I will stick to that.
At the moment I am having 52ºC running Prime95 with the CPU stock cooler and one Chasis fan set at turbo speed. If set at standard speed it gets up to 59ºC. But, hey, we are talking about Prime95 and other programs running at the same time.
I am glad I made this choice of this CPU, good for everything even hard gamig (with a good graphics card ex. 5770 and above) at a budget price like 63 €.
But, to answer to your question, at last, yes, you can take it way beyong 4000 MHz, you just have to watch the temps and of course know what you are doing... here you are these links that will help you a lot...
http://www.overclock.net/t/1002985/athlon-ii-overclocking-guide-beginner-friendly
http://www.clunk.org.uk/forums/overclocking/22106-core-i7-overclocking-guide-beginners.html
http://www.youtu.be/watch?v=5a7AxZJ7tWc&feature=related
Good luck!!!!!!!!!!!