Looks like I will be doing three across in my Outback later this year: 1 FF, 1 RF, and one bucket, which means new car seats! My oldest will be 4 at the time... should I get a harnessed booster when it is time or replace her bulky convertible with something slimmer? I don't really know if there is a difference at that point in usage. She'll be in the middle and responsible for climbing in and out herself (will still get help buckling the harness).