I've got about the same age gap between my kids (My son turns four in May and my daughter turns one in March) and I love the Sit 'n Stand. I picked mine up at Target, where I was able to get one down off the display and look at it first. I like that it folds down flat enough to fit in my somewhat small trunk and still leaves some room left for groceries! It's also reasonably lightweight and decently maneuverable, although the wheels are not designed for off-roading, per se, so I tend to stick to the sidewalk.
I had a Graco carrier from my son that I used again with my daughter as I knew it hadn't been in any accidents and was still in great shape. I set the Sit 'n Stand up in the setup they gave for Graco and it fit just fine. The instructions are a bit confusing - go with common sense as you set up the carrier bar and you'll probably get it right the first or second try though. The carrier clicks onto the crossbar and the other end rests into the seat.
My son loves sitting in the back, facing me. With the carrier in place, the basket underneath is a bit awkward to use until you realize you can snap and unsnap the sides to get at your stuff. Overall, I've found it's the best stroller available for those on a budget.
A friend of mine has this gorgeous side-by-side umbrella stroller she uses with her twins that she swears by - it has harness straps to keep infants in, and is big enough for her 3-year-old too, maneuvers like a dream and has decent enough wheels to offroad. However, it was something like $700 to buy!
Hope that helps!