Hi J.,
At four months, your little guy will probably be just fine if you have him sit with you and your dh; you probably don't really need to pay for the extra seat. He probably won't be crawling yet so he won't be as restless as if he were already mobile.
If you have him as a 'lap baby', then you might want to pack a couple little rattles and interesting things for him to play with. New is better because he won't get bored with it as fast.
I second the bottle or binky, or if you are nursing, just nurse him and he'll probably do great. Try to have him sucking and swallowing on something during take-off and landing, because the pressure changes can hurt their ears and that can really make them cry. A couple things for you to remember -- change the little dude's diaper right before you are getting on the plane, since some airplanes don't have changing tables and those that do are very cramped and usually kinda dirty in my experience. Maybe bring a change of clothes for him and change of shirt for you, since he might spit up.
I used a Baby Bjorn with my little guys at that age, and it worked great. It gave me free hands, and baby was happy being close to mama. Also my dh could use it if he wanted.
HTH! Have a great trip!