Gotta love those big boys huh? I have a almost 4 year old boy who is 40 lbs and a little over 42 inches. We have the McClaren umbrella. We bought it last year just for just for Disney, once I tried it I actually handed down my giant Graco Quattro stroller to my neice. Now we only use the McClaren, at the mall, at Disney and at our local theme park. It is great and goes to 55 lbs. It folds up nice and small for the busses and airplane. It is a little pricey but well worth it...esp if you can find one used. And with the current stroller rental price increase...it will probably be cheaper for you to buy than rent.
This is the model we have and we actually took the straps off, he just sits when he's tired and walks when he's not. Works great!
Maclaren - Triumph