Yeah, I feel a little strongly about that one. We flew last Feb with three kids ages 5 mo's, 1 almost 2 and a 5 year old. It was a nightmare for all of the reasons Sybah shelstah mentioned. Flights were delayed, almost tow year old was scared of the noise the plane made and cried hysterically when we got on, diaper leaked and it's a pain changing a whole outfit in those tiny bathrooms, ran out of milk, kids were stuck where they were--first in the boarding area of the very boring Midway airport, then in their seats as we sat and sat and sat on the tarmac--it goes on.
Then in Dec, we drove (this is from MN, by the way) and it was a wholly different trip. We left in the evening after DH had slept most of the afternoon and drove through the night. Our roughest time was stopping for breakfast. A sit down restaurant was not a good choice at that time. (Although on the way home it worked fine.) DH will tell you that 3:00 in the morning was not a good time either since the two yo and six year old were wide awake and chatting his ear off when he just wanted to be alone and drive, but hey, at least they were happy.
On the way down we stopped a lot. The kids needed to get out and run a bit, so we went to most of the state welcome centers and a few rest stops, had a picnic lunch at a park etc. Stayed at a hotel that night and drove the rest of the way the next day. Had lots of new things to give the kids and lots of story tapes and new kids music from the library. On the way home, everyone was pretty happy so we just kept going. Ended up getting home at 11:00 at night the day after we had left at about 5:00 in the evening.
For the 4 month old--or is that a 4 year old?--Well, if the baby needs a bottle, The Right Start sells a bottle warmer thingy that wraps around the bottle and plugs into the cigarette lighter. Very handy and doesn't take up much space. Also, toys that you can hang from the handles or hooks in the back seat keep the baby entertained.