A.B.
It's hard on everyone when Dad is away a lot. Noticed something similar with my (now) 5-year-old daughter. There can be several reasons for this:
a) He's mad that Dad is away so much.
b) He is seeking your attention.
c) He is having a developmental problem.
At 4, he should be able to answer if he's sad that Daddy is not home. If that won't or can't change, I've explained that to my preschoolers and then have tried keeping them busy. Some might disagree with that, but it helps my family. We three would go on fieldtrips, do crafts, invite over friends. Special outings are used as incentives for good behavior, like parktime.
He could be seeking Dad's attention or YOUR attention. You wrote that you have a 15-month old, who I'm sure requires a great deal of Mom right now. If Dad is the one who keeps up with him and you're with the baby girl, your son might feel like you love the daughter more. Some children will take negative attention over no attention, and some don't have the words to express their anger, disappointment, frustration with having too little time with a parent. I try to get them to realize that positive attention is so much more enjoyable. You could issue a game or activity that is done only when Dad is away, like baking or crafts or board games. Not sure when Dad is out, but if your daughter goes to sleep shortly thereafter, that makes the time even more special because it's now "Mommy time."
I have to add that routine and consistency are so important for preschoolers. I didn't realize how much so until my daughter went to a preschool camp. Knowing what's happening next gives them a sense of control and order. When Dad leaves, your son's sense of order could be off balance, and he might be having a hard time adjusting.
If it's a developmental problem, you will have to talk to pediatrician and/or Early Intervention team to assess him. Before that, though, try to get him to see that he's the oldest and is needed in the family. He sets the tone for the family. We used Dad's absence as a time to teach our son responsibility. Of course, he didn't understand what that meant, but we kept teaching it. We taught him that as the oldest his behavior taught his sister how to act. When Dad had to leave, he'd say "you're the man of the house when I'm away." (Of course, he couldn't really do much, but I noticed he'd stick out his chest and say "Aye, aye, sir.") Talk to your husband. It must be a male thing. My son is now 10 and is very helpful and mature. (Not perfect, by any means.)
And, try to get Dad in the habit of spending one-on-one time with either of them. My children are older and I now make the time extra special so the other child doesn't feel left out. That child is having time with MOMMY. (Even though I'd love to just take a nap!) It's a balancing act. But, once my children were all filled up and reassured that no one had abandoned them, they were better able to separate and play on their own, which is when I really can take a break.