AMD Phenom II X4 955 Deneb 4.00ghz

yes, it's capable ! also depend your CPU cooler

no. it is impossible...lol. yes of course it's capable, it all depends on the details. Is it a C3 stepping (not required, but helpful)? Do you have a decent motherboard that's capable of a decent OC? Do you have a good CPU cooler? Do you have good RAM? Do you have adequate case ventilation? have you read the numerous guides here on Tom's?

3.8 Ghz is majority of C3 chip (it can easily reach). 4.0 is posible but you need a good cooler for that....

It is possible but unlikely most the Phenom II's (945,955,965) OC closer to the 3.4Ghz to 3.8GHz range. However if you do your research and have optimal cooling and the extreme patients to constantly adjust settings in your BIOS its obtainable