What's the difference? is one better or are the older versions only kept for the sake of keeping? I saw that 4.5 is no longer available. I'm using 4.6.2, and it seems to be fine...
You can see a list of the major changes here. If you're happy with the version you're currently using, I suppose there's no hurry to upgrade though.
The GCC project generally maintains two stable release branches. Currently those branches are 4.6.x and 4.7.x.