Most airlines only allow one infant per adult passenger, so you may not be able to fly with them both. Some allow two if the older infant is in a car seat, the airline will provide a list of approved restraints for various aircraft.
I’ve flown with one as an only adult, he was almost two, to be honest he slept really well, it was a night flight both times and I did everything I could to keep him awake in the airport. Just remember toys etc can’t be noisy or bulky, which can be tricky as so many toddler toys are either huge or noisy. If he needed them I had a new stacking cup set, but they were ruberised so if he whacked them together they weren’t noisey. When he was awake he very much enjoyed having his own TV, that kept him very entertained, I just had to make sure he wasn’t kicking/hitting the chair in front, or rocking back in his own chair too much.
I imagine the four month old will be much easier, our daughter flew in April when she was five months old, she was in a chewing phase and apparently spent most of the flight chewing giraffey, she did however hate the cot thing, so my husband had to lay her on the floor when he had his in flight meal, as they were sat at the bulk head and he had booked her a seat, there was lots of room on the floor for a blanket, toys etc without getting in anyones way.
Just remember if any of them need changing, or if you need the loo you’ll be in a tiny plane toilet with all three of them. I’ve done it with me and just on toddler and that was enough of a nightmare, so it would probably be best if you don’t need to use the toilet while you’re on the plane to reduce toilet trips.