Flying East Coast to Hawaii

First and foremost, don't panic. It sounds really scary, but actually babies and small children (before they can walk) are WAY easier on plane journeys than older ones! Fight the urge to bring too much stuff - the important things are extra clothes, diapers, wipes etc (as someone else mentioned) Also, remember that the best cure for aching ears on take off and landing is sucking - so have a bottle or breast ready for both. The likelihood is that s/he will sleep for most of it (as s/he does at home) - and people are actually really helpful generally when you have a little one with you. I don't know what security is like with formula though - I hope they aren't to difficult. Enjoy Hawaii!!!

Take extra clothes on the plane for you and baby, more diapers than you think you will need, and lots of baby wipes. Oh, and chocolate and ear plugs for your neighbors just in case your baby isn't happy. :)

There are flights now direct from JFK to Hawaii. Perhaps a quick shuttle to JFK and then the flight direct to HI would be easier than transferring at LAX. This way, once you are settled in for the long flight, you can just relax the entire trip rather than have to pack up 1/2 way through, deplane and start over again.

Otherwise, at 4 months, you'll be fine. As long as a baby isn't walking, life is pretty easy.

Good luck and have fun!

You are nursing, right? Then it's easy :)

You might think I'm being flip, but I'm not. At 4 months, your baby doesn't expect to get up and walk around all the time, and when you are nursing you don't have to worry about getting bottles through security, warming bottles on the plane, etc.

Here are my recommendations:
-Get a window seat. Then you only have to worry about accidentally flashing someone on one side while nursing. If you are flying with another adult, put that person in the middle seat and you'll have a lot of privacy.
-Nurse during takeoff and landing. That will make sure the baby's ears adjust. Don't rely on a pacifier. You have to swallow to pop your ears, not just suck.
-I carried my baby in a ring sling on the plane. I used it when I was sitting, because it helped to give my arms a break from holding the baby, plus I used it like a light nursing blanket over my shoulder and the baby for privacy/modesty when nursing.
-a handful of onsies for the baby (for this long of a flight, I'd take 4, more if your baby is a spitter), a light blanket, and twice as many diapers as you think you will need.
-in your carry-on, take an extra set of clothes for you! Then when the baby spits up on you, you won't have to sit in a bad smelling shirt for 5 more hours :).
-some colorful, relatively quiet toys - rattles are fine, but out of courtesy for your neighbors, please leave the talking Elmo at home unless he has an off switch that you plan to use. I always liked toys that I could hook to strings of those plastic rings that link together, because I hooked the top ring to my ring sling so when baby threw/dropped the toy, it couldn't go too far. If you plan to take a car seat, you can also hook the rings onto a car seat strap.
-many planes have changing tables that fold down in the bathroom, but not all. So change the baby immediately before you get on the plane. I've always been able to find a spot to change diapers, even when there isn't an official changing table, but it is harder on the plane.

I didn't fly that long with a 4 month old, but my daughter went to Hawaii at 6 weeks and slept the whole time on the plane, at 4 months she was still sleeping on planes, but most of our flights were around 3 hours or less.

If breastfeeding, prepare to feed a lot more than usual. Babies tend to cluster feed on flights (probably out of boredom). If formula feeding, pack extra formula and while it maybe scrutinized at security you can bring it through.

Also, for some reason my daughter loved to poop on planes. I always found myself changing more diapers on the plane than I would if we weren't flying.

Feed or give a pacifier during take off and landing to help with their ears.

I also carried my daughter as a lap baby and see did fine sitting on my lap. I also found turbulence to be my friend. The bouncing of the plane kept my daughter asleep.