Honestly? Flying with a 4 month old is cake. Flying with an *18 month old *sucks. 4 month old can cry sure, but mostly is happy enough if you offer a bottle/the boob. And with luck will sleep. An 18 month old can spend an entire 6 hour flight from SFO to JFK awake and jumping in your lap and fall asleep in the last 5 minutes of the flight during the decent into JFK. Trust me on this one.
The trip that stands out for me though was when I took my then-3 month old and then-2.75 year old from CA to NY alone.
The flight attendants were very sweet and helpful, and brough snacks at off-times. And a nice lady in the row across from me held the baby every time I took the older one to the bathroom. The older kid watched TV the whole time - thank you JetBlue!
Nursing/bottle feeding on takeoff and landing is a must. I also brought a jar of earplugs to offer around, thinking it would buy me goodwill but mostly people just grunted at me or ignored me completely, so don’t get your heart set on definitely getting anything out of the effort.
In your carryon pack more diapers than you think you need and then put in 3 more. Same with wipes. Put in a new shirt for yourself and 2 changes for the baby. If you cloth diaper bite the bullet and go with 'sposies just for the flight (do a test run to check for diaper rash first). If you are bottlefeeding or nursing more often for soothing, you’ll go through more diapers and probably have more poops or runnier poops to deal with. Bring butt balm of some sort. Also a changing pad, the optsions for changing a baby in an airprit and airplane aren’t too fantastic.
Dress in layers, both of you - if the plane is held on the runway (god forbid) it might get hot in the plane, and also sometimes it can get hot just while the plane is flying. Also I wore my baby in our Moby, which was easier for me than the ergo, YMMV. But hodling a baby for that long also can get hot.
Grab a pillow, or two as you are walking into the plane, just steal from someone else’s seat (or ask the flight attendant). You’ll be glad to have the cushioning for your arms against the armrests, holding the baby that long.
I was never really able to walk up and down the aisle in a plane - there is always a drinks cart in the way and it is so narrow it doesn’t make for good pacing, but again YMMV.
Either way, even if the baby screams the whole flight, remember that it is finite, eventually you will get there and go on with your life. And someday when your kids are older, you’ll see a young mother with her baby on an airplane and think to yourself “thank god I never have to do that again”, and turn back to your book. 
Good luck!