M.C.
The 3 and 4y will not need carseats on the plane. You can however bring your own car seats for when you land. They can be checked for free.
Personally I would get the baby his own seat. That way your hands and arms are open to helping the other kids. As for what carseat would be allowed for him, you'd have to check with your airline.
A portable DVD player will do wonders!
Take a pack of diapers for the bag on the plane, but otherwise wait and buy a box of diapers when you get there.
For the baby, you will be allowed to have a few cans of sealed ready-made formula (if you are bottle feeding). For the pre-schoolers and adults, all liquids have to be purchased after security, but you are allowed to bring whatever dry foods/snacks you want.
Try to keep the carry ons to one bag per kid.
As for packing. I am a bit weird. Instead of packing a whole suitcase for each person, I pack a suitcase for a place. We 4 people went on a week trip with 3 hotel stays. 4 suitcases. For the first stop, I fit all of the clothes for the first stop in 1 suitcase. Hubby thought I was crazy. Then when we got to the first hotel I only had to unpack one suitcase. When we were done at that hotel, that suitcase was full of dirty clothes and moved to the back of the trunk. When we got to the second hotel I got the 2nd suitcase out. Again, only one suitcase to unpack. When we were done with that hotel, THAT suitcase was moved to the back. When we got to the 3rd hotel, I got out the final 2 suitcases. When we left there, all clean clothes went into one and dirty into another. Then when we got home, the clean suitcase went upstairs for unpacking and the others to the wash room.