Is it normal to lose 4.5 pounds in 10 days? by only eating 900 calories and not exercising. or is it dangerous?

Relative: That a wee bit restrictive. If you are morbidly obese, rapid weight loss is often not difficult. The first weight lost is often water. The liver contains glycogen which carries water. When that is expended water will go with it. If you are not obese & continue at a rate of loss this rapid- it may be too fast. If you are female - have you considered up'ing to 1, 200 calories. Talk to your doc.