We are going 10/4 with our 6 month old and 2 year old. We are taking a double for the parks and most of our activities and a single for just the baby for shorter outings (we are driving, so we have room- if we were flying, I'd leave the single behind).
We typically find that our 2 year old changes her mind often about if she wants to ride or walk. When we have 2 strollers, I find that we are often trying to keep up with the baby stroller, the empty toddler stroller, and a walking 2-year old. If she wants to walk and we have the double, one of us can push the stroller and one can keep up with our walking DD. So we find it easier to just take the double.
I recently purchased a Maclaren Twin Traveller. It was expensive ($350), but for lots of reasons, we decided it was best for us. It has full recline for the baby, an acceptable sunshade, and it folds compactly with a carrying handle.
The trouble with a lot of doubles, particularly the tandems (front/back) and the inexpensive side-by-sides, is they are so heavy and bulky that they are trouble on the busses. We had a Graco DuoRider and it was difficult to get in and out of the car . . . I couldn't imagine trying to carry it on to a bus. I sold it and bought the Maclaren, in large part because of my concern about the WDW bus. My DH thinks I'm crazy and would argue that the bus hassle isn't worth the $100-200 dollars more for a different stroller . . . but everyone has their own opinion. We use our double for other outings, so I could justify the purchase. If your 4 yo really doesn't ride much anymore, you may have a harder time justifying the double purchase.
If you decide to get a double, Combi makes a great double (Twin Savvy) that is lightweight, folds in thirds and has a carry strap that allows you to carry it like golf clubs. I really wanted this one, but DH and I are both tall and the Maclaren handles are much higher. You can find them on-line for about $250, but you probably don't have time to order one, so you'd have to find it locally. Peg Perego also has a lightweight side-by-side that runs about $300 at Babies R Us. Its nice and lightweight, but the sunshade isn't very big.
You might also want to consider a kiddy board. Its a platform with wheels that you can attach to your single stroller so your 4 yo can catch a ride. Here's a picture
http://www.kiwi-kid.com/lascal.html
I don't know a ton about them (my 2 yo isn't ready yet), but you can find lots of people who swear by them on the iVillage Stroller Board:
http://messageboards.ivillage.com/iv-ppstrollers
HTH . . .
-Leslie