I also have the JOOVY, but it's the ULTRALIGHT version. I bought it simply because I did not want to push around these other humongous double-strollers in-and-out of small spaces (shops, crowded places etc.). I really do love this stroller. And it really IS ultra light. You can EASILY stroll/maneuver around with both children using one hand if your other hand is preoccupied! It's really that light. And I'm a small person, only 5"2"/100 lbs. So it's even light for me.
However, the only drawback is that the toddler seat doesn't stay in place and tends to push back when my 2-year-old sits in it. I wish they would have created some sort of a contraption that helps to keep the seat in place. Hence, what I use are binder clips to clip the side bars where the seat is attached -- to prevent the seat from moving back (the clips block it from moving further back). Not sure if I'm making sense. Other than that, I really love it.
As far as having the toddler sleep, I'm not sure if this is possible, since I've never done it. But if your 4-year-old can sleep sitting up, then I don't see a problem with placing a pillow to the side of him. There should be enough space.
In regards to your concerns about his height, my daughter is 37 inches (she's 2 years and 3 months) and she fits in it fine. Hope this information gives you a general idea.