Hi,
After having about three cheaper 'character' backpacks fall apart within a year, I finally bit the bullet and got my son the more expensive LL Bean rolling backpack about 2 years ago and it's still going strong! We live in MA and it has lived through all kinds of weather. It can also carry it on the back as well, just make sure to tuck the straps back in if it's being rolled afterward; if not, the bottoms of the straps can get worn away (they do warn you about this on the backpack). Although this happened to us once, all I had to do is call the repair hotline and they sent a new part free of charge. Their designs are pretty subtle as well, so I didn't have to worry about my son growing out of that year's trendy character. It also fits fine in his locker, which he has to share with another child. I would have your 4 year old try it out in the store since the rolling backpacks are on the larger side. Something to remember with all backpacks...don't embroider your child's name on it! It gives out the name to strangers who can then pretend to know your child. Better to do initials to differentiate from other backpacks.