Is it possible for me to get to a size 4 or 5?

I'm 16 and 5'11 and 145 pounds. I just found that I can fit a size 7 because I used to be a 9(my butt is kinda big but not really more medium big). I know I'm not overweight and I wasn't when I was a 9 but I wanna lose more. Is it possible? 34b boobs
Is it possible for me to get to a size 4 or 5?
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