I tried the 4 and felt it ran a bit big, i'm 34a-28-26, 5'6" about 130lbs and expected to take a 6. the 4 was ideal.
love the concept but thought the material was too stiff to be a 'swing' tank. the pleats in the back poke up when you move or bend as if you've got little horns coming out of your back. i think a chambray, like cloth and stone produces might have rendered this super cute design better (imo).
it has a trapeze shape, i wouldn't plan on tucking it in.
finally, you mightn't want to