L.N.
there are kid size airplane earplugs (make sure it's ok to use on 4 month old). i have used those since my kids were 1. always great
when i traveled with infant kids i always made sure the next bottle they took was during take off and then again during landing. it's important that they are swallowing during that time. never had problems with ears although one of mine is notorious for having ear problems otherwise.
good luck