interesting. i am glad you mentioned diet and exercise is needed. i do see a flaw in your spell however, what if someone is 300 pounds and wishes to be 120? i doubt the spell is so powerful and rapid that he/she will loose 180 pounds in four days. not to mention how dangerous loosing so much weight in such little time would be. i feel it would work if you were wishing to slim down the one or two dress sizes the tight outfit would be.
side note, while body image is a big problem and many people not only judge based on appearance, but also feel they will be judges/disrespected/unloved if they are not skinny as a rail. there are those who are happy and healthy even though they are overweight. i understand when casting this type of spell you must look at all the positive, but skinny does not equal happy. happiness is not a number, many will gain confidence with weight loss, but basing your joy off what others see you as is a bad thing to base on, and a spell cannot change your mental image. [i'm not mad, just wanted to point that out to those who feel the only way to be beautiful and accepted is to be thin is a bad way of thinking and you should have more personal reasons why you wish to change your appearance.]