I have seen brie cheese labeled 2X and 4X and would like to know what that means.
I think you're referring to "double creme" and "triple creme" brie. In this, the milk is enriched with added cream until the butterfat content is double or triple the normal level before the cheese is made.
According to French law, this works out to 60-75% butterfat for double creme brie, and 75%+ for triple creme. Note that's the percentage of the dry matter in the cheese, excluding the water contained in the cheese, and not total volume. Here's a good post explaining the differences. In general, these tend to be much creamier and richer than "regular" brie.