Wow...this is a tough one. Traveling with one child on the plane alone is a challenge, but two...you will be an all time hero. I have been traveling alone with my now 16 month hold since she was 6 weeks. She has gone on nearly 10 airplane trips 2 of which were international and she is taking another next month.
Anyway, I think your situation is not completely hopeless. Although your 4 year old is the one that seems to be scared I think that I would start now to try and turn the experience into a game for her. I would make her your helper for the entire trip which includes packing for the plane activities for both her and her sibling (ie. snacks, games, favorite doll/stuffed animal, etc.). While in the airport she can be responsible for different aspects of the trip which may include carrying the bag of treats or helping to fold the stroller, just something. While on the plane you can have several distracting activities planned like reading books and singing songs. They recommend that children are sucking or chewing on something during take-off and landing (i.e. juice, milk, pacifier, gum). The flight attendants are usually very sympathetic and helpful to make sure that you and the kids have an easy time during the flight. When the plane is actually taking off I would make sure you're talking to her and maybe even singing some sort of fun song.
Once the plane is in the air you may even ask a flight attendant to show her the cockpit to visit the pilot to keep her engaged and to see how safe and not so scary the experience really is.
I'm sure everything will work out just fine...GOOD LUCK!!