Flying with 4.5 year old and 20 month old - Carseat Advice

Sorry if this is rambling, but I am really stuck on what to do here! We will be flying at the end of the month and then renting a car at our destination (first time flying with both kids, although the oldest has flown many times.) My 4.5 year old is 43 inches and 43 lbs. My 20 month old is 32 inches and 34 lbs. We have 3 carseats at the moment - the Graco SmartSeat (4.5 year old's primary seat), the Graco 4Ever (20 month old's primary seat) and the Cosco Scenera, which has been our travel seat in the past.

4.5 year old has never flown without a carseat, but I think he meets the requirements now so I've been thinking of giving that a try and just having him sit on the plane seat and use the lap belt. I then would use the Scenera for the 20 month old...I do hate rear facing it on the plane but he isn't tall enough to forward face. We can make it work though. However, that still leaves me with needing a seat for the 4.5 year old when we get our rental car. The SmartSeat is a beast and taking that anywhere is out of the question (it's not even FAA approved, so it's never left our car before.) I could bring my youngest' Graco 4Ever and have 4.5 year old use that (which will also be a pain to carry), but from my research it sounds like it's not a good idea to check carseats and I don't want anything to happen to it...can I gate check it then? Or should I bring it on the plane and have my 4.5 year old use it on the flight? I'm not necessarily opposed to that option, but I would prefer to sit with both kids and have my husband on the next aisle seat and I know we'll have to split up if we use 2 carseats. I don't particularly want to purchase a 4th carseat, but I guess that could be an option too... Any advice from the experts or those who have been in this situation?

I'd bite the bullet and take the 4ever for your 4.5 year old to ride in on the plane. The hassle of trying to safety check a car seat definitely outweighs the hassle of taking it on the plane with you (in my opinion).

I wouldn't consider buying a 4th seat (personally) but an Evenflo Maestro would work (he has 7lbs until he outgrows the harness and then can be used as a booster) and it is pretty budget friendly and more lightweight than the 4ever.