You seem to be sold on the idea already, but if you're willing to entertain some thoughts from a parent of a just-finished-sixth grader, here goes:
First, entering fourth grade, she should not have so much stuff on a daily basis that she needs a rolling backpack -- if you are concerned that she'll have to haul textbooks back and forth, don't be. Most schools now either use online textbooks, send printed ones home to stay there for the entire year, or use them in the classroom only. She likely will not be carrying as many books as you think she will. In fact, I've noticed that kids who have huge, heavy backpacks often are just hauling a ton of graded papers, notebooks, library books they should have turned in weeks ago, etc. -- in other words, good organization and regular clear-outs would mean they didn't have to carry so much each day. If she is organized and you help her develop good habits of removing library books to keep at home, giving you her old papers, etc., that lightens her load and teaches her organization at the same time.
Second, have you checked if the school has any policy that forbids rolling backpacks? Some schools ban them for a number of reasons, including size and safety (I can't tell you how many times I've watched kids stumble over other kids' rolling packs because kids are looking up as they walk -- which they should do! -- and don't see the floor-level backpack in their path; I've tripped over these things myself.). Be sure that it's OK to have these. And just because you've seen some kids with them does not mean the school approves. Our elementary school allowed them, but our school for next year does not -- largely because they do not fit in the lockers and next year the kids have lockers for the first time.
Third, have you had her really try these and see how heavy they are? She is going to have to lift it into a locker each day (especially if the locker is on top of another locker) and these things can be pretty heavy when empty -- imagine adding stuff to their empty weight. If she tries them in a store, she should load them up to see what it would really be like to heft one around.
Fourth -- 13 inches wide? That's very narrow indeed. You might be finding ones that are 13 inches wide, but if fully packed, how deep from front to back is the backpack? You give dimensions for height and width, but the loaded backpack might need to be in the locker at times, and will it really fit front to back, not just in width and height?
As you can tell, I'm not a fan of these things. A good-quality backpack with good, padded straps, loaded properly (heaviest items closest to the child's back) and regularly cleared out, carrying nothing but what's really needed, and most of all worn properly so it doesn't injure the kid's back -- that's what I'd recommend. But if you are sold on the rolling pack, be sure it's not going to be so heavy when loaded that it's a pain, and be sure the school permits it.