If the 1 year old is not ticketed, there may not be an empty seat for the car seat. Again, I always flew with my child in a babysling, so even with turbulence, he/she wasn't bouncing away. And truthfully, I have logged lots and lots and lots of flights over the years as a coast to coast traveller, and although there were some rough moments now and then, turbulence never injured anyone on one of those flights. Yes, it COULD happen, but statistically, not that much of a concern. I'm not trying to argue that it couldn't happen, but sometimes you need to put it into perspective. Kids die on the playground too, or from stray bullets, or falls from bicycles or choking or---you get the idea
The booster seats that most 4 1/2 year olds use in cars are not approved for airline use, so unless you are hauling a Britax Huskey, (plus another full sized car seat for the baby in this instance), you wouldn't be able to use it. >
But getting a ticket *is an option. I've travelled frequently with my children, and done a bit of international travel as well. While I agree that accidents can happen anywhere, I prefer to control the things I can--using a carseat is one of those things.
We used our son's five point harness booster seat on planes until he was 5, at which point the lap belt fit him properly and he was over 40lbs. I just wouldn't feel comfortable taking a chance with a one year old not being restrained on a plane or in a car.