Flying with a 4 year old... can we buy her a booster seat for the rental car?

[ourbabygirl]

The last time I flew with the kids we had DD (3 at the time) in a Cosco Scenera that we have for flights/ rental cars, and I think DS might have still been in his infant seat. He's 2 now, so we're going to have him in the Scenera, and I'm wondering if I can get a small booster for DD for the rental car so we won't have as much heavy stuff to bring (we'll have at least a few suitcases, a stroller, a pack-and-play, etc. to check as luggage).

FWIW, she's currently in a Sunshine Kids Radian in one of our cars, and a Britax Frontier in the other car. She's at least 45 pounds (as of October) and 44 or 45 inches. I'm way out of the loop on car seats with 'older' kids, but I'm wondering if there's a smaller/ lighter seat/ booster seat I could get for times like this. It seems like some of the 4 year olds I know ride in booster seats in their own cars.

If it seems safe enough to you, is there one you would recommend? And a good (cheap) place to order it from?

Thanks for your help! :)

[KDsMommy]

For a 4 year old I'd probably opt for a high-back booster like the Graco Turbo Booster. It comes apart in 2 pieces so you could put the back in your checked luggage and strap the seat part onto a carry on or backpack.

The Bubble Bum also gets recommended quite often for travel. I have no experience with it, but do know that it's a backless booster.

[Neatfreak]

My oldest kid has a Bubblebum, and it's the most awesome thing to travel with! I've sat my four year-old in it for fun, and the fit of the shoulder belt is okay in some vehicles. The lap portion is fine.

[twowhat?]

I plan on getting this for my 4yos for the rental car when we fly next year:

http://safetrafficsystem.com/ver4/

Only drawback is the price...but it would be nice to keep on hand as an alternative to a second set of car seats.