Hi there - first of all, what everyone has stated above is correct and it's not my intention to argue their points. I'm just here to offer the solution we use when booking hotel rooms (about five to six times a year) because we're also a family of five who find it challenging to travel in a lodging world that seems so intrinsically geared toward families of four.
Our family (two parents, one teenager and two schoolagers) travels with our own inflatable mattress and pump for our teen. Our mattress, an Aerobed single, is light and easy to pack (the pump, not so light, but the total added weight is less than 8 lbs). We book rooms with either two beds that are at least doubles or one double/queen/king plus a pull-out sofa for our littlests. Our teenager sleeps on the inflatable. At least in our case - problem solved.
I should add that we're not hiding anyone when we present ourselves at check-in - we come into the lobby together and leave together and the size of our party is clearly visible. We also do not take advantage of breakfast in the morning for more folks than we paid for - we pay for an additional breakfast instead. The one or two times I've needed to explain the number in our party, I stated that we wanted to keep our family together and that ticking off 'party of 5' wasn't an option during online check in (those who've tried booking a room for 5 people probably know what I mean). It's never been a problem and folks at the hotel seem to understand the idea of parents wanting to keep their children together in one room. We haven't yet encountered the clerk or manager who asked us to purchase another room, though we realize that might someday happen.
A couple of disclaimers: 1) I haven't tested this theory on Disney-owned hotels as we stay off-property, and 2) this strategy definitely doesn't work in Europe, where rooms are smaller, breakfasts are often built in and hotel proprietors tend to be compensated based on the number of people staying with them as opposed to the number of rooms they sell. If you were going to DLP, for example, it would make sense to find an apartment (which is what we did on a recent trip) through Booking.com or some other means as capacities can be limited, especially in large cities, to just 2 or 3 people
Best of luck on your vacation, I'm sure it will be special no matter what.