What cpu cooler do you have?
Not all Phenom II x2, x3 or x4 can hit 4.0ghz. So far a had 4 x2 and 1 x4 that could hit 4.0ghz out of like 20 cpus. Most of them hit 3.9ghz and this is with a 890FX mobo and D-14 cooler.
my cooler is Antec H2o 620. (fits in my case very well and cost about the same as a good air cooler)