I travel solo with twins. With those ages, buy a mifold for the 4yo, so you only have one car seat to manage. Get a strap to attach the car seat to a rolling carryon.
https://www.amazon.com/Convert-Luggage-Airport-Stroller-Carrier/dp/B06XTT17QC
I would probably skip the stroller and just bring a baby carrier but I almost never used the stroller anyway. What I would have done was backpack/baby carrier on my body, carseat+rolling suitcase in one hand, 4yo holds my other hand. Then you only need to let go of the 4yo if you are checking additional bags and if that’s the case only until they have been checked.
If you bring a stroller, plan to gate-check it and make sure that you can reliably steer it with one hand. Another option I have seen work well is a folding wagon. Not always the comfiest seat for the kids, but you can put a toddler and a backpack in it and have one free hand for the suitcase/car seat. Then 4 yo has a backpack with their toys and books for the plane.
I also pack really light. If you are traveling within the US, just order diapers/wipes/formula to your destination via amazon (if visiting family), or arrange a grocery pickup for the day you arrive and stop on the way from the hotel.