You are not a bad person and its only natural to feel this way. SD sounds like a princess Chucky on the loose and I don't blame you for not wanting to be around her. Have you spoken to her father and been honest about your feelings towards her? What does he do when she acts this way? Does he parent her and give her age appropriate consequences? He has to take a stance and teach her how to behave.

If you haven't had the talk, you need to NOW because its only going to get worse the longer he doesn't address it.

Don't feel bad, my skids aren't terrible and I still make excuses to be gone and away from them. The upside is that at this rate I'm going to be in the best shape of my life because I'm always at the gym trying to get away!

Most of us find it hard to love kids that aren't ours, especially ones that are brats. Even if you do end up caring for them, it's still not the biological bond, and anyone expecting that from you has never been in your position and doesn't understand that It's an unrealistic expectation.

I don't have any advice, but others here will. Welcome!

At 4 her parents should be the ones pulling her back to reality when she enters her hysteria.  She sounds in desperate need of parenting, limits and rules.  Its no wonder you can't stand to be around her and have doubts.  They are well founded.  He should be ashamed of letting a 4 yo rule the  home with her out of control behavior.  If he doesn't start parenting her now, she will be like this at 14, 16 , 21 and so on.  Does she treat all adults like she does you?  Is that acceptable in her father's eyes?  It doesn't really matter what BM thinks, what happens in his house is up to him to manage.  Kids can learn to behave in different envioroments as long as expectations and consequences are spelled out and dished out consistently.  

You have a man problem.  Plain and simple.  

You would be smart to walk away if you are not married, and if your partner does not have any interest in raising a well-behaved child. These problems will only get bigger and nastier if she is this bad at 4. It's not a phase, it's a spoiled child who is the victim of poor parenting in BOTH of her homes.

You stated "shared custody" in profile, so I'm assuming 50/50 at each home. 

Right now the little tyrant is the product of her parenting (or lack of).  Where is Daddy when she is ripping on your curtains? Are you the only one home with her during these emotional fits or is SO just there and ignoring the tantrum? 

There is nothing 'cute' about  a 2-4 yr old who goes through the 'terrible years'. Perfectly normal for you to find it unappealing and well, unlovable. She's not yours and there isn't that unconditional love/bond between you and the acting out kiddo. Don't feel poorly about that. Frankly, when my GS was like three I use to silently mumble to myself 'kiddo, it's a good thing I love you or I'd right now be tossing you out the window' . LOL. He would deliberately try and push my buttons. With him it wasn't curtains, it was my wallpaper I had at that time in part of my breakfast nook. Little brat would look at me, put his fingers right at the seam and dare me to defy him. 

If you're staying an going to keep trying you need to sit Daddy down and inform him your expectations. You know, the ones where you expect him to parent his child. It's his job as the parent to teach. He also needs to be sure his child has respect for you. Nope, she isn't going to get her Mommy if she rips at your curtain , she's going to get a time out , or a favorite toy taken away or whatever the going rate is for ill behavior. 

If he isn't taking your concerns seriously, there likely is little hope for things getting better. If she's a terror at 4, just think of what it will be when she's 14. She'll be an intolerable monster. And it will be no one's fault but her father and/or mother. It is hard for little ones to go from one home to the other as different homes have different rules, expectations and parenting styles. But with that said, if Dad is consistent with her in your home she can indeed learn this or that doesn't 'go' in Dad's home. 

Kids do go through phases. My SO's younger daughter, who is 3.5, was a sweet, always happy, cooperative child from the time I met her a couple years ago until about a month ago. Lately she's been much brattier, whiny and less cooperative. OTOH, his older daughter, who was much brattier than the younger one since I met them, has turned a corner at age 6 and is much better behaved.  Neither of them approach what you're describing, though.

I'd give it some time and watch closely how she's being parented.  If her parents are being consistent, expecting good behavior and imposing consequences when she falls short, she'll likely come out of the bratty phase and it's a matter of time.  If not, you might want to think about moving on. Other people's children (even people you love) can be annoying at the best of times.  Expecting a relationship to be healthy, happy and even survive for 14+ years when a skid is ill behaved and not parented is expecting a lot.

Ok, for starters, she is 4. There is nothing nice about 4 year olds. I have 2 bio kids and 2 step kids, I got together with my husband when his youngerst daughter was 2. She was sweet and we bonded, till she turned 4.
Honestly, I didn't like my own kids when they were 4. 4 year olds suck, at 2 years old they say "NO!" at 3 years old they say "NO" and tell you where to go and at 4, just FORGET IT, they are horrible creatures! Good news is that at 5, they start school and it starts to turn around.
My daughter and step daughter at 4 would refuse to wear clothing picked out for her, even on special occasions and would only wear what she picked out, would pout for photos or refuse to be in them, would ask me to do her hair and then look at in the mirror and rip it out screaming at me because it was "wrong" AHHHHH
I now have 4 teens ranging 19 to 13 years old and I would take and live with a dozen teens! before I sent 1 day negotiating with a 4 year old!
Sorry, I'm not to proud to admit, I just simply don't care for that age!

Its not SD,, ITS DH, he is not parenting her, not giving her boundaries.  This will never end, until DH stop it.  I just hope BM will support DH in this and not play into SD hand and crying.  If BM supports SS all is lost 

Same here. I, too, have ran away just to get away. It’s been a horrible experience. In my defense, my SO has ran away too or has sent her away. 4 years in, its not any better. I’ve had to almost completely disengage from SD. I do blame bad parenting. She is of age that she knows right and wrong; but still bad parenting is a huge part of her behaviors. I’ve talked with SO about it, more than once. It’s done not much good. 

I relate with you with the "not wanting to be around" part!! I distance myself when my boyfriend 's daughter comes around. Sometimes her existence annoys me. I know it's bad to think this way but I'm just being honest!! I spend more time at the gym! But the kid doesn't even have a bed time schedule which annoys me as well. She will go to bed whenever sometimes at 3am and who's staying up until 3am??? Especially on a work night! Have you gotten over these feelings? Are you still with your boyfriend?