Hi A.,
It sounds like to me that your 4yr old hasn't got too much he can control in his little life. He's been the center of attention for 3 years, now he has to share mom and dad and now dad is about to leave. I suggest you spend some alone time with him, leave the baby with a friend or family member one afternoon a week if you can. He needs to know that he is still special to you. Dad needs to do some serious prep for his deployment. Get a tape recorder and have dad read bedtime stories into it (at least enough for two weeks). That way you can sit with him and go through the book while dad reads on tape and you don't get the same one over and over and over. Dad can also take a tape recorder and record tapes. That way since little guy can't read or write, they can exchange tapes. Be sure to keep them and he can listen to them when he misses dad. I also saw on TV where you can make your own stationary with pictures copied onto regular writing paper at places like Kinko's or Office Depot. Just tape pictures around the border, make a color copy and that way dad gets updated pics everytime you mail him a note. It is also affordable (I think it costs about 75 cents to make 1 color copy). It also becomes something to comfort dad when he is lonely for you and the kids. You can also make a chart and give big brother stars for "excellent big brother behavior" When he earns a specified number of stars he gets a treat. Usually treats given at the equivalent of their age. age 4 gets a treat after 4 stars. It doesn't have to be big. Sometimes just a sticker or special movie time with mom. Find the good things he does like picks up the baby's toy or hands you a diaper. Remember that it takes 4 positives to 1 negative to change behavior so even if you can only say something like "what a good big brother you are for talking nice to baby sister" and give him a star. In the beginning you will have to LOOK for things to be positive about because if you are like me I get stuck in the negative "don't do that, put that down, stop that". You have to make an effort to shape your child's behavior and it has to be consistent. If he truly does something like hitting that is unacceptable then he goes to time out. It will take you a long time to get that in place if you haven't been doing that but it will be worth the effort in the long run.
Hope some of that helps.
C.