AMD says it can go to 1.42 for daily OC'ed use. Personally I keep mine below 1.375, and after that, If i can't hit the desired speed, then i just go with that. Mine is running at 3.9 @ 1.35. I'm happy out with that. FOr the extra heat generated by going to 1.4 4.0ghz, it's just not worth it for 100mhz increase.
Run coretemp and check what your VID is. This will give you an indication of what your max voltage should be. If you go over it, the lifespan of the CPU may decrease over time. So instead of maybe 10 years, you might get 8 years @ 1.4