Technically, I am not sure about what you say, Leon4.

In fact, inside the "levels" directory, every possible level set is represented by a sub-directory named "packX" where X is the level set (for examble, LP3 is composed by "pack6", "pack7" and "pack8", LP4 is composed by "pack9", "pack10" and "pack11").

Curiosly, "Angry Birds Seasons" levels directory is containing the following sub-directories : "pack12", "pack13" and "pack14". So, this means that it is the following of LP4.