Always go for the latest version, which at the moment of writing is 6.3.
The earlier versions like 4 are still avalaible but they are full of bugs and security hazards, which are solved in later releases. If you are a 1337 h4x0r and particularly enjoy headaches, by all means go for an earlier version. Why else would one want to waste hours killing bugs Chances are, when you are on this blog you are far from being 1337 So stick with the latest version, it’s the safest plus there is more active support to find online.
Then there is the bit thing to deal with, CentOS comes in a 32-bit version and 64-bit version. You can read about the differences between those versions here.
Since I’ve have it in my head had to take some Red Hat exams, I am installing the 64-bit version. According to Michael Jang you do need a 64-bit system to prepare for the exams. So 64-bit it is