I agree with K S that your daughter could be reacting to the new baby even tho this seems like the same behavior as before you were pregnant. And to your assessment that this could be a defiance issue. K C's suggestions could help your daughter become less defiant if defiance is part of the issue.
I also think that your daughter's personality is different than your son's and therefore she needs a different approach. Some kids are born more stubborn than others and it helps to make them think things are their idea. One way to do that is to give them choices. You can choose to help pick up your belongings and keep only a few things out at a time or you can choose to have me put up everything but a few things and we can trade toys when you want to change activities.
At four you might be able to have her help with the decision of what to do. Tell her that it is not acceptable to have everything out in the room at once and ask her how she would solve this problem. Give her suggestions from which to choose. This conversation is time consuming and ongoing and I'm not sure that it would work for a child that is so energetic in her actions. If you don't already include her in decision making it will take time to teach her how to be involved and to make good choices as well as show her that you trust her.
I also suggest that you find a way to prevent her from being able to get into her drawers. She could probably figure out how to circumvent the baby proof latches if she hasn't already. Perhaps you could have her help you install them and discuss how you know she can still get into the drawers but that they're a reminder that the drawers are to stay shut and full.
My grandchildren tend to get out all their toys at once. I think this is normal for an inquisitive impatient child. My daughter's solution has been to remove the toys that they don't seem to play with but that they still take out and clutter up the floor. Often they do this because they're looking for a particular toy. I added to this by putting small toys in smaller plastic bins and large toys into a large plastic bin. Stuffed animals go into a large box. My grandson plays with his play tools often and so they go into a bin by themselves. By doing this the kids don't have to go thru all their toys to find the one that they want.
It's my guess that if there's enough stuff in the room to take hours to clean up the mess then there is too much stuff in the room or you are trying to be too organized about cleaning up.
You could try stopping your daughter before she's got everything pulled out by changing her activity to include you. If she has never been able to pick up her toys by herself I suggest that you stay with her patiently helping her until she learns to do it herself. Make picking up toys a game. My grandkids liked singing the "time to pick up song." I showed them how they could hold their shirt up and put lots of small toys in the "pocket" to carry them to the toy box.
My grandson, who is now 4, wouldn't even help pick up his toys until a social worker showed his mother how to teach him. She sat him next to the toy box and handed him the toys. When he was able to do that consistently, she raced him to see who could pick up the most toys. She kept up a pleasant chatter during the whole time, praising him for everything that was helpful. She'd describe what he'd done and not just say good job. Then she just sat with him while he picked up the toys. Putting them away was simple. Dump them in the box, one for toys, and one for books, paper and markers etc.
He still doesn't always put away his toys. It's guarenteed he won't pick them up if someone just says pick up your toys. Instead we say, let's see how fast you can get those toys in the box. Or let me help you get started. When you're done I'll put on a video of your choice. All the time being present even tho we're doing something else. That sort of thing.
All of this takes more time than just telling a child to do or not do something but it does have the potential of teaching them a more successful way of acting as well as the ability to eventually when they're older to do it entirely on their own. A 4yo still has a short attention span.
When I read that you work at home doing medical transcription I wondered if you are also dealing with your 4 yo at the same time as you're working. If so it's possible that she needs more supervision than you're able to give her and/or she is trying to get your attention while you are understandably withdrawn and focused on your work. Perhaps changing your schedule would help. For instance talk with her about how you are going to work uninterrupted for an hour and then the two of you will do something of her choice for a specified period of time, perhaps for only 10 or 15 minutes. Talk with her about what she is going to play with. Be involved with her for a brief time at the beginning of the hour.
If you aren't able to supervise closely then I think that she should have only limited access to a few things at a time. Rotate the toys so that she remains more likely to be interested for a set period of time. Check in on her frequently, even tho it's difficult to interrupt your train of thought. If she's within hearing distance it might help to just call her name and make a comment so that she feels that you're paying attention to her.
Perhaps focusing on only her and her needs, patiently teaching her for some hours, while your son is in school, might help. Talk with her letting her know that you understand that it's difficult for her to keep her room picked up and ask her for ways in which you can help her. Let her know your difficulties and how she can help you. Focus on and spend time developing yourselves as part of a team. This will take a lot of time and patience. It's definately not easy and very time consuming at first. And it may not work if some other issue is causing the behavior.
If you don't already, perhaps you could work less during the day and your husband could help more in the evening so that you could work while he's with the kids.
I'm basing my suggestions on my experience with children when I'm distracted, because I want to do something else, while they take care of themselves. Preschoolers just aren't very good at being ignored. They will get your attention in any way they can. It's up to us to teach them positive ways to get your attention.
I've found it difficult to strike a balance between getting "my work" done and meeting their needs. I made the mistake with my daughter of requiring that certain things be done a certain way and thus lost her co-operation early on. She was one of the most stubborn children I've known. Now she has two children nearly as stubborn as she was. I'm learning new ways of doing things right along with my daughter.
I wish you success. Changing her behavior will be difficult and take time and patience.