Are all of your guests staying on the property? A 4-hour-plus gap is hard to fill completely with activity unless it's a theme park. It might just be easier to let people go back to their hotels to hang out there and relax. Otherwise it makes for a really long day of activities, which might not be enjoyable if people are well-dressed (as I'm sure they will be since it's your wedding), and the ladies probably won't be wearing sneakers, etc.
There are tour-like activities at the parks, but you would have to spend money per person to buy the tour, and you'd have to be sure that everyone has park admission. There's not really an activity that's long enough to fill the gap, and if you do something that takes up half the time, then people are left with a little bit of time that they can't do anything with, so I vote for no activity.
I hope that's not a downer! I just think then that all your guests can show up to dinner nice and refreshed and ready for a fun night of eating and socializing, instead of showing up tired and starving.