What am I supposed to weigh?

I'm 5'5", and now I weigh 144 pounds. I'm on a diet right now, and have been for a while - I used to weigh 190. I know I have a small frame, my wrists are the exact same size as my ten year old sister, (btw, she is pretty thin, not fat or chubby...) and my mom says I have birdie ankles. Haha? I'm 15 almost 16, and wear a size 7-8 right now. I wanna be a size 4. In June, I'm going on vacation, and I'm gonna shop at the Mall of America, so I want to be happy with the size I am when I go. I hate being a size 7-8. So, does anyone know how much I should weigh to fit into a size 4? Thanks in advance!:)