She's driving me CRAZY!

I am 20 years old and I have a very vivacious 4 year old, with one on the way.  I love my daughter very much, but I am literally about to rip out my hair.  I have had a pretty good handle on her, and I don't let her get away with misbehaving, but I cannot keep up with this.  
Today, for example, we ate breakfast, and then I asked her to go wash her hands (something we do after almost every meal.) not even 2 seconds later, I hear my Fiances razor turn on!  I freaked and ran back there, she had climbed onto the counter, grabbed the razor, and had it inches away from her head!!  She has gotten in trouble for touching it before, and she new that it was wrong, because when I asked her what she was doing, she coyly said, "I don't know."  She has been acting out ALL DAY, EVERYDAY!!  I don't know if she's doing it for attention, knowing that this new baby is coming, or if this is just a phase, but it is making me terrified to be having another girl in a couple of months.  
My fiance works two jobs, so I have her all the time, and nobody to help me discipline... I need advice.  Please someone tell me I'm doing the right thing by getting her into trouble, or am I being a bad mom? How do I get her to mind me again?

My little girl seems to get into the most mischief when she's bored. Maybe a new activity or something to keep her focused. If not, the time out chair it is! I also try to catch my daughter doing good things, and praise her. Your doing fine and I don't blame you for being worn out. The fact that your asking for advice shows what a good Mommy you are.

It is really hard. I am 23 years old, and I have a 9 month old son as well. I have been where you are...actually, I am there every couple of days. She is at the age where she is going to test you. My 4 year old nephew is the same way. Just stay on top of her. Do you yell at her a lot? I used to yell at my daughter a lot and it wasnt working. I stopped yelling at her and started to stay calm and I have a lot less problems with her

Quoting ChaseMeElizabeth:“ It is really hard. I am 23 years old, and I have a 9 month old son as well. I have been where you are...actually, ... [snip!] ... daughter a lot and it wasnt working. I stopped yelling at her and started to stay calm and I have a lot less problems with her”

I don't yell but my tone changes, and she can see it on my face that I'm frustrated with her.  I try to include in her all my activities (ie. paying bills, doing dishes, running errands.) But its getting so hard now that I don't want to take her anywhere!  Yesterday we went to Sears, and when we were looking at some new clothes, she ran up behind me and pushed me, hard enough that I lost my balance!  Is it so bad that she's jealous of me paying attention to clothes!