I'm w/ Veronica. Skip the car seat and use the seatbelt on the airplane. She'll be much more comfy - especially on such a long ride ..... when she will likely be up and down quite a bit :). I flew to Europe when my daughter was 2 and we spent a lot of time walking back and forth to the bathroom, to the kitchen area, to the beverage cart, etc. Little ones get restless on those long flights and need to move. So long as they are not disrupting other passengers, it's just fine, IMO. Flying is not like riding in a car and you don't have the same 'crashing traffic' risks where you need the 5-point harness. The seatbelt is mostly to keep you snug thru turbulence, breaking, sharp movements and such. The planes, lap-belt should work just fine for that - especially at age 4.