Have you thought about hiring a bonded babysitter to come to your hotel room? Your 4-year-old could play with the babysitter while your DS14 could either hang out there with his sister or perhaps go to an arcade or something (some of the resorts have them -- like the Beach/Yacht club, which your DS could walk to from the Dolphin. The Dolphin or Swan might have one -- I don't know for sure, but I know the BC/YC has one).
Another idea is if you have dinner at one of the parks, you could let your DS go to that park and do some attractions, then meet with you after dinner. For example, Disney MGM is right there and you could have dinner at one of the MGM restaurants while DS rode rides or whatever, then meet up outside Fantasmic and the three of you could go together.
I'm sure it's difficult with the age difference, and I don't think it's fair to ask DS14 to watch the four-year-old if he doesn't want to (and if he did, I'd pay him to do so), but I also wouldn't want someone else's 14 year old son hanging out with my four-year-old daughter in a child care situation just because the 14-year-old's little sister was there, too. As someone posted, at 14 years old, your DS is old enough to be a babysitter himself. You can probably talk over the options with him and ask what he thinks he'd like to do while you have dinner, but I do think that sending him along with the four-year-old to "kiddie camp" is not something that would work well.
-Dorothy (LadyZolt)