Traveling with kids very much depends upon traveling with them in a way that they will find enjoyable, and in a way that won't leave you a wreck.
Fortunately, a 4 month old has a simple definition of "enjoyable": regular feeding, regular changing, sleep and snuggles. Expecting about 14 hours of sleep a day, in spurting naps of 2 to 3 hours each, you can easily manage a long trip. I've done it successfully: just expect to be feeding, changing, and snuggling every few hours.
Planning is essential here: have everything you need at hand when the babe wakes. Change, feed, burp, snuggle/talk/read, then tuck in. Repeat for the flight.
Yes, the pressurization may make babe fussy. Try to avoid travel if your babe has an ear infection, as that narrows the air path ways and intensifies the pain. If babe fusses during equalization, give a bottle: the sucking motion lessens the effect just like chewing gum for an adult.
If you go with multiple flights, consider the stress you will feel lugging babe around concourses, meals, and any checkpoints you have to cross. Personally, I find the logistics of connections more troublesome, because you have to keep your entire party synchronized. If you have to change a diaper and miss your connection, what then?
In my mind, for a 4 month old, it's better to hunker down, establish a routine, and enjoy the long flight.
Now when babe gets to be a toddler? Split it up, trust me!