Re: Airline/Travel car seat situation..
Since he (your 4yo) has outgrown the CARES, your options are carseat or plane's lapbelt. At his age and size, the plane's lapbelt should fit him (but won't always on every kid in every plane). Since you'll have a seat for him on the other end, I'd just use the plane's lapbelt.
S, mama to my 3rd grader, K (9/07) and my 1st grader, e (7/09) and married to my best friend, E (2007)