I would keep him in the one that he and you are familiar with. 4-5 hours with breaks is really not that long. He'll be fine.
If you don't already have a stroller, I would buy the one you would buy if he wasn't in a carrier anymore, since you will probably get more use out of the stroller over the years. Or, borrow one from a friend (or pick up a cheap one at a resale shop) that you can snap the car seat in. Though I rarely did that with mine...my kids liked being able to spread out in the stroller without still being confined to their car seat.
-L.