Not all Phenom II's can reach 4Ghz, also that motherboard has limited BIOS options for overclocking and a rather weak 4+1 phase vrm. Either of those are the more likely culprits that are limiting your overclock. You can of course always try enabling/disabling load line calibration and turning off/on power saving features as well as dialing it back to maybe 3.8-9Ghz.
Even if it isn't the psu, I'd say it's still a good idea to replace it with something more reliable. You don't want to mess around with cheap bundled psu's.