A 6A is an outside cabin. A 4E is an inside cabin. It depends on whether you want a window in that example. Sometimes an inside cabin can be a good choice if it is on a deck closer to what you like.
For the most part, when dealing with standard cabins an inside is an inside, and an outside is an outside. They are the same size cabins. The category really indicates which deck the cabin is one. So a category 4A inside is on a lower deck than a 4E, although the cabin is identical. But the 4E will cost more - the price for being on a higher deck. Oddly enough, the cheapest cabins are on the lowest decks yet they have the best ride if the seas get choppy.