You will be able to stay in the park as stated you will just need to trade your ticket in for a writst band anytime after 4pm. However food service will not be open between the "offical" park closing time and the start time of the party. Because they are trying to clear the park of the day guests that are not attending the party, the stores and food will close promptly with the park closing. For instance, with the Halloween parties going on now, the park closes at 6pm sharp... all merchandise, food, etc. They generally have a section of the park that will be open for rides (last year was Tomorrowland and Fantasyland) they will send everyone with the wrist bands over there so that they can clear the day guests out easier. So yes you can stay in the park, but plan your food prior to the park closing or after the party start time...