Is getting your 4 month old a seat on the plane an option at all? I only ask because we opted to have our 3 month old as a "lap child" back in April, and it was super stressful. We had some turbulence along the way and I was sooooo nervous during the entire 3 hour trip.
We are not taking that chance for our upcoming April trip and I bought him a seat. He will be secured in his car seat. Sucks to bring extra stuff, but his safety is worth it to me.
We flew very minimally when our girls were babies, so I must have forgotten how sucky it is to have the "lap child" for a 3 hour flight. Never again.