Hello. I don't know the exact differences but I'm pretty sure about one thing....
The higher up the number the better, for example, you're asking about cats 4,5,6 etc... CAT 12 is a Penthouse suite...
Also, the letters I think indicate the location of the cabin.
For example cat 4a is probably the least popular (but may be the cheapest)
but a cat 4 c might be a better location, like mid ship.
That's how I understand it, but wait long enough I'm sure you get a more detailed answer, and probable more accurate too.
(since I'm still a newbie)