How do I terminate my parents custody of my 4 year old niece?

My heart goes out to you in your effort to try and help your 4-year-old niece. I am also fairly confident that your parents mean well and want to do the right thing for your niece but fail to recognize their limitations and its impact on a 4-year-old in their household. I am also fairly confident that you have spoken with your parents about their limitations and your belief that you can now provide your niece with a better home setting. If you have not done so, I think that you need to sit down with them - if for no other reason but to show your maturity and your willingness to address the issue constructively. They may not see it the same way and may not respect your efforts but you need to have that conversation before moving forward with any alternative plan. Presuming that the conversation accomplished nothing, then you need to decide if you want to speak with the state division of Child Protective Services. You can file a complaint with the DCPP and offer to take your niece in with you and provide all of the reasons why you feel that it is an unsafe/unhealthy environment for your niece and simultaneously why your home setting is more appropriate. I also presume that your parents are receiving governmental benefits for caring for your niece and may be resistant to give her up but maybe you can agree to a trial plan setting where they continue to receive those benefits for a period of time to ensure that your home setting is working.