the 4.0Ghz is it's built in boost, if you go into the BIOS (or use a program) you can turn that off and increase the multipliers/voltage on your CPU to get a true overclock, the rest I agree with jay2577 above. Read some guides on how to proceed or you may shorten the lifespan of your CPU.
Also there is a small difference between the 8350 and the 8320 besides factory overclock, the 8350 is often higher binned, aka the silicon of the chip is usually of a bit higher quality, but it honestly makes no difference in the end for most users, and the only people who care about it are people who want the highest clock rates out of their systems, purely for milestones or bragging rights. The 8320 will O.C within 200-300 Mhz of the 8350 O.C'd almost 100% of the time