J.H.
My family and I were living overseas in Japan, so I travelled frequently with my, now, 11 month old. Unfortunately on the first trip we bought a seat for him and also used the snugride, but since he mostly slept on me, it seemed a waste of money. After that we would check both the carseat and stroller (they will put it in a plastic bag for you) and I used the baby bjorn carrier when he was small, and now I use the Ergo carrier, but any similar baby carrier or sling will do. I find it to be the BEST thing on an airplane because it makes getting on and off the plane easy, plus with my son, if it was time for his nap, or if it was an overnight plane ride, I would go into the bathroom on the plane and sway him back and forth until he fell asleep. And then he would sleep in the carrier on me. We have taken about 6 or 7 plane rides with him now and it has worked everytime. I hope this helps, I know that travelling with a baby can be stressful, but at 4 months he will mostly sleep, which is a good thing! Also if no one has told you this yet, plan to feed your baby on take-off and landing because this way he won't feel the change in pressure and you won't have a screaming child on your hands. Good Luck and Hope this helps!