We've cruise a few times with our little one (2.5 years old). Each time, we've stored her car seat on the desk in the stateroom. We just placed it on the far side of the desk, up against the wall. It was out of the way enough there that it didn't cause us any problems. We just put a yellow luggage tag on it and dropped it with the rest of our luggage. You'll hear differing opinions about whether or not to do do this--yes, they treat it just like the airport folks do--tossing it around and it may even be on the bottom of a stack of luggage. 3 cruises with the car seat, and it's still in one piece. If the 4 year old is in a booster seat, have you looked into the Mifold Grab-n-Go Booster? Also, you can buy a strap that attaches to your suitcase to lug the car seat through the airport--the kiddo can even ride in the car seat while it's attached.