No appetite for weeks..one meal per day not hungry at all..working out daily lost 4 kgs in almost a week ..what's wrong?

Many possibilities: You may be hyperthyroid, have an underlying infection or tumor, may be depressed or have some other condition which has increased your basal metabolic rate while somehow suppressing appetite. You should see a primary care physician and be thoroughly evaluated asap. Good luck and best wishes for a rapid improvement. Do not take this to an er;.