I would do it, the 4mo is still immobile and if you're bf it's easy to shove a boob in they're mouth and they're ok! If they're FF hopefully with a full tummy they're likely to sleep
Depends more on your 2yo... Is he likely to sit still and watch cartoons & sleep?
Like I said I'd do it, but we fly a lot so my kids know what to do. The first time we went though wasn't all that fun.