Honestly, I really do. You're counting on that seat to protect your child in the event of a car crash... which requires it to be in proper working order. But when you give the seat over to the custody of the airline, you have no control over what they do to it. I put this video in another thread, but just for the sake of inclusion --
Carseats being thrown before loading onto the airplane
Granted those appear to be checked seats and not gate-checked seats, but gate-checked seats are carried to the ground and loaded into the plane just as checked seats are. There's a thread here on the transportation board (I think it's the Delta one) where a poster gate-checked her stroller and the airlines broke it.
At least if the airlines break it visibly, you know that the seat needs replaced... there's also the possibility that there's no visible damage, but there may be invisible stressors that are cause for seat to fail to protect your son in a crash. That, IMO, would be the worst case scenario. I'm cautious when it comes to kids and seats, but I'm just so hesitant to trust that a piece of potentially lifesaving equipment will not be mishandled.
Is your younger son an older 4 or 5 year old? If he's not a young 4, not terribly petite, and relatively mature, I might consider training him on how to ride in a booster at home and using it on vacation. That way you can pack the back(s) in luggage and just carry the booster bases on the plane -- stow them and use the airplane seatbelts. Or, there are other options, too -- like a travel vest, which operates like a 5-point harness but folds for easy transport -- but I can certainly understand not wanting to invest in something like that for a trip. If you really feel most comfortable with him in the 5-pt, still, then I think I would deal with the hassle and carry the lightest weight seat you have for him onto the plane.