We are going through a very similar situation with our daughter, for the identical period of time- two weeks. Further, she too has a younger sibling, and I am 5 months pregnant with our third. Though I continue to be stressed about this, I am less so now that I have discovered that an enormity of other four year olds seem to hit this same glitch.
She's getting less of the one-on-one time she has been used to, because I'm so exhausted this pregnancy I practically nod off during breakfast. I also suspect that I am grouchy lately, while during the earlier part of my pregnancy I was patient and even-tempered. She's extremely excited about the baby, though.

She recognizes she's going through a tough time. She told me she wants her sadness to go away. We asked what would help-She says she wants to go to things for kids that aren't lessons but just a bunch of kids having fun (She takes swimming lessons). So, we enrolled her last night in some fun stuff, but they don't start for over a month. Already, this morning, she's less excited than she was last night. Normally she holds her excitement for a long time. If the sadness keeps up despite all the upcoming fun things in her life, I wonder if we take her to see someone? I really can't begin to say how normally happy she is- smiling, loving, laughing, joyous.
My husband, too, has a rough time dealing with it. I find her behaviour right now worsens if we get angry (which we sometimes have to do). When she immediately switches back to her ol' wonderful self, I give her smiles and hugs. Even when she's acting as miserable as possible, I an usually soften her with a hug and as positive a comment as I can come up with. He (my husband)is really receptive to comments, though. I will say to him quietly later that  his criticsms might be snowballing the problem, and give an example or two. He always says sorry to her, if he feels it's appropriate, and I'm so appreciative- I don't know what I'd do if I also had to lock heads with him over this.

Wow, this message is volumes longer than I intended. Sorry! Thanks for reading my first post. My future ones will be shorter!

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I am having a very similar problem with my son and he will be three in May. He has always been a "perfect child" until recently. I took him to a child play therapist and even pulled him from daycare. He has chnaged back to himself in just a couple weeks. I dont get the drastic changes he has made either and he is a wonderful child at home until he gets around dad. His dad works alot and only sees him a couple hours a day so he acts like a complete different child around him. He is angry,mean,rude, and talks to him in a terrible way. It his way of getting dad's attention and it is hard to deal with because dad'd feelings get hurt so badly and he has NO idea how to deal with it. All I can gather of the whole thing is that he feels he is lacking the attention that he needs and tries any way to get it. It is a very hard struggle with a child to help them understand we do our best, but they can't understand. We are just dealing with it day by day and dad is trying to give him different attention all that he can. It seems to help to settle him down by really giving him that one on one he feels he doesn't get. I guess to try to help, I am saying maybe your childfeels the baby is a "threat" in a way to HIS attention and maybe try to involve the child in the excitement of the baby more, even if it seems silly to you soemtimes. I mean to us cartoons they enjoy are sometimes "dumb' to us but they are the world to them...little things are so big to a child..we forget that in the big world...........

I am having trouble with my 7 year old going backwards  with his 3 year old sister he takes her barbies toys and everything she gets even if he get somthing he wants her toy to he does not care what it is. He throws fits all the time.. He was soo good before she came along what can I do.. She is a sweat girl and his a good boy does he feel left out? help

Such an acute change is disposition is often brought on by either illness or by some change in the environment, particularly the social environment. What do you suppose might have your son unsettled? Is the upcoming birth the only change in the family or other important areas of your son's life? What is he saying about the baby's coming? The behavior toward his younger sibling is some indication that this is what has him unsettled, but don't assume this. Perhaps also your husband needs to question how he is interacting with your son. The important thing is to be decisive regarding setting limits, but to maintain equanimity at the same time. Perhaps your husband himself is unsettled about the upcoming change in the family? How is he reacting to the prospects of having another child?