Pretty much, personally, I like the 6 in the park, but I know people (most of the time smaller kids) that like the 4 (or even hunt for an old #2), I also know people who ride 8s and 10s in the park. They can all be used, it just comes down to, do you want a stiffer or softer boot.
I just kinda answered this in another thread, but it mostly depends on how big you are and the amount of ankle flexibility you have. Weight is the easiest- if you are a park skier and weigh 250 pounds the park #4 or #6 will be too soft for you. If you are 120 pounds, the #8 or #6 will be too stiff for you. In the end, you need to get fit and try them on to see how much you flex them.