I actually posted a question about this before we went to Disney in Jan. My dd was 9 months old when we went, so it was a little different. I already KNOW this, but it's hard to not worry.......the anticipation is usually worse than the actual process of airplanes, shuttles, hotels, etc. LOL! In our case, the flight was at a pretty convenient time as far as nap/feeding. I had to mix up a bottle after we went through security - in which I asked the vendor for a bottle of water that wasn't refrigerated. Once on the plane, I waited until take-off and gave her the bottle. Since I had both her binkies and her blankie, once we were going, she fell asleep in my arms. The plane thing worried me, too, but I wasn't going to miss out on an opportunity to do Disney (funded by the in-laws!!!) because it was inconvenient to take the baby on the trip. She didn't get sick, slept well in the packNplay at the hotel, and we really didn't have much issue, at all! My older kids, plus their cousins, had such a blast! We've now said we are going again when my now 15 month old daughter is older.
A couple of things...the hotels can "order out" to a local pharmacy if you need anything that you forgot to bring, lost, etc. Since my daughter had weend herself at 6 months, she was on bottles. I didn't bring enough formula and we let the hotel know as we headed out in the morning and the formula was waiting for us that evening.
Not sure if you already know about this stuff, but if not, the babyswap is awesome! My hubby, MIL or FIL would stay with the babies (there was a 3 year old cousin with us, too) while the rest of us went on a ride. Once the first group was off, we switched and the one who stayed behind got to go - so the kids got to go twice, and you didn't have to go back out and wait in line! Mostly, we got fast passes for the bigger rides, like Tower of Terror, etc.
So, basically, it will be a great time for your family. The baby will probably sleep when he gets tired and eat when he's hungry, just like at home. Take along plenty of hand sanitizer, but it's not like the 4 month old will be touching stuff, like a toddler might. If I had to take my daughter, now, it would be much harder, since she wouldn't want to be in the stroller, would want to pick stuff up off the ground....ew. I think it will be a good time to go, before he's mobile, and doesn't respond very well to "No, ick" :) Good luck and have fun!!!